feat: added migrations docs and a migration script

docs/upgrading_to_v2.0.md

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+# Upgrading to v2.x
+
+The v2.x release of _google-network_ is a backwards incompatible
+release.
+
+Because v2.x changed how the subnet resource is iterated on, resources in Terraform state need to be migrated in order to avoid the resources from getting destroyed and recreated.
+
+## Migration Instructions
+
+-   Upgrade to the new version of this module.  
+
+if you run `terraform plan` at this point. Terraform will inform you that it will attempt to delete and recreate your existing subnets. This is almost certainly not the behavior you want. For example:
+
+```Shell
+Terraform will perform the following actions:
+
+  # module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork will be destroyed
+  - resource "google_compute_subnetwork" "subnetwork" {
+      - creation_timestamp       = "2019-10-02T08:40:26.282-07:00" -> null
+      - enable_flow_logs         = false -> null
+      - fingerprint              = "f8LZx006zY4=" -> null
+      - gateway_address          = "10.10.10.1" -> null
+      - id                       = "us-west1/simple-project-timh-subnet-01" -> null
+      - ip_cidr_range            = "10.10.10.0/24" -> null
+      - name                     = "simple-project-timh-subnet-01" -> null
+      - network                  = "https://www.googleapis.com/compute/v1/projects/dev-xpn-networking/global/networks/simple-project-timh" -> null
+      - private_ip_google_access = false -> null
+      - project                  = "dev-xpn-networking" -> null
+      - region                   = "us-west1" -> null
+      - secondary_ip_range       = [] -> null
+      - self_link                = "https://www.googleapis.com/compute/v1/projects/dev-xpn-networking/regions/us-west1/subnetworks/simple-project-timh-subnet-01" -> null
+    }
+
+  # module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[1] will be destroyed
+  - resource "google_compute_subnetwork" "subnetwork" {
+      - creation_timestamp       = "2019-10-02T08:40:26.292-07:00" -> null
+      - enable_flow_logs         = true -> null
+      - fingerprint              = "wOwStN9lK-Q=" -> null
+      - gateway_address          = "10.10.20.1" -> null
+      - id                       = "us-west1/simple-project-timh-subnet-02" -> null
+      - ip_cidr_range            = "10.10.20.0/24" -> null
+      - name                     = "simple-project-timh-subnet-02" -> null
+      - network                  = "https://www.googleapis.com/compute/v1/projects/dev-xpn-networking/global/networks/simple-project-timh" -> null
+      - private_ip_google_access = true -> null
+      - project                  = "dev-xpn-networking" -> null
+      - region                   = "us-west1" -> null
+      - secondary_ip_range       = [] -> null
+      - self_link                = "https://www.googleapis.com/compute/v1/projects/dev-xpn-networking/regions/us-west1/subnetworks/simple-project-timh-subnet-02" -> null
+    }
+
+  # module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork["us-west1/simple-project-timh-subnet-01"] will be created
+  + resource "google_compute_subnetwork" "subnetwork" {
+      + creation_timestamp       = (known after apply)
+      + enable_flow_logs         = false
+      + fingerprint              = (known after apply)
+      + gateway_address          = (known after apply)
+      + id                       = (known after apply)
+      + ip_cidr_range            = "10.10.10.0/24"
+      + name                     = "simple-project-timh-subnet-01"
+      + network                  = "simple-project-timh"
+      + private_ip_google_access = false
+      + project                  = "dev-xpn-networking"
+      + region                   = "us-west1"
+      + secondary_ip_range       = []
+      + self_link                = (known after apply)
+    }
+
+  # module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork["us-west1/simple-project-timh-subnet-02"] will be created
+  + resource "google_compute_subnetwork" "subnetwork" {
+      + creation_timestamp       = (known after apply)
+      + enable_flow_logs         = true
+      + fingerprint              = (known after apply)
+      + gateway_address          = (known after apply)
+      + id                       = (known after apply)
+      + ip_cidr_range            = "10.10.20.0/24"
+      + name                     = "simple-project-timh-subnet-02"
+      + network                  = "simple-project-timh"
+      + private_ip_google_access = true
+      + project                  = "dev-xpn-networking"
+      + region                   = "us-west1"
+      + secondary_ip_range       = []
+      + self_link                = (known after apply)
+    }
+
+Plan: 2 to add, 0 to change, 2 to destroy.
+
+------------------------------------------------------------------------
+
+Note: You didn't specify an "-out" parameter to save this plan, so Terraform
+can't guarantee that exactly these actions will be performed if
+"terraform apply" is subsequently run.
+```
+
+### Manual Migration Steps
+
+In this example here are the two commands used migrate the subnets created by the `simple_project` in the examples directory.  _please note the need to escape the quotes on the new resource_.
+
+-   `terraform state mv module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[0] module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-01\"]`
+
+-   `terraform state mv module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[1] module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-02\"]`
+
+`terraform plan` should now return a no-op and show no new changes.
+
+```Shell
+$ terraform plan
+Refreshing Terraform state in-memory prior to plan...
+The refreshed state will be used to calculate this plan, but will not be
+persisted to local or remote state storage.
+
+module.example.module.test-vpc-module.google_compute_network.network: Refreshing state... [id=simple-project-timh]
+module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork["us-west1/simple-project-timh-subnet-02"]: Refreshing state... [id=us-west1/simple-project-timh-subnet-02]
+module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork["us-west1/simple-project-timh-subnet-01"]: Refreshing state... [id=us-west1/simple-project-timh-subnet-01]
+
+------------------------------------------------------------------------
+
+No changes. Infrastructure is up-to-date.
+
+This means that Terraform did not detect any differences between your
+configuration and real physical resources that exist. As a result, no
+actions need to be performed.
+```
+
+### Migration Script
+
+1.  Download the script
+
+    ```sh
+    curl -O https://raw.githubusercontent.com/terraform-google-modules/terraform-google-network/master/scripts/migrate.sh
+    chmod +x migrate.sh
+    ```
+
+2.  Run the script to output the migration commands:
+
+    ```sh
+    $ MODULE_NAME="test-vpc-module" ./migrate.sh
+    terraform state mv module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[0] module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-01\"]
+    terraform state mv module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[1] module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-02\"]
+    ```
+
+3.  Execute the migration command
+
+    ```sh
+    $ terraform state mv module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[0] module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-01\"]
+    Move "module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[0]" to "module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-01\"]"
+    Successfully moved 1 object(s).
+    $ terraform state mv module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[1] module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-02\"]
+    Move "module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[1]" to "module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-02\"]"
+    Successfully moved 1 object(s).
+    ```
+
+4.  Run `terraform plan` to confirm no changes are expected.

scripts/migrate.sh

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+#!/usr/bin/env bash
+# shellcheck shell=bash
+# Output Terraform Commands to migrate to new subnet config
+set -e
+set -o pipefail
+
+if [[ "$MODULE_NAME" ]]; then
+  NAME=$(terraform state list | grep ${MODULE_NAME}.google_compute_network.network  | sed 's/.google_compute_network.network//')
+  for x in $(terraform state list | grep ${NAME}.google_compute_subnetwork.subnetwork); do
+    ID=$(terraform state show $x | grep id | grep -v ip_cidr_range | awk '{ print $3 }'| tr -d '"')
+    echo "terraform state mv $x ${NAME}.google_compute_subnetwork.subnetwork[\\\"${ID}\\\"]"
+  done
+else
+  echo "MISSING MODULE_NAME: MODULE_NAME env var is required"
+  exit 1
+fi