GCP auth metadata config

[tyrannosaurus-becks]

This PR adds the ability to configure the metadata that will be included on during login. The auth Metadata field drives what's written to audit logs. The auth alias Metadata field drives what's included in Vault's identity system.

A high rate of change to fields on alias metadata can cause performance issues at scale because each change triggers a storage write. The fields included in alias metadata here are identical to what was already previously included, and we simply add the ability to change them.

Command-line walk-through

$ vault auth enable -path=gcp vault-plugin-auth-gcp
Success! Enabled vault-plugin-auth-gcp auth method at: gcp/

$ vault read auth/gcp/config
Key             Value
---             -----
gce_metadata    [instance_creation_timestamp instance_id instance_name project_id project_number role service_account_id service_account_email zone]
iam_metadata    [project_id role service_account_id service_account_email]

$ vault write auth/gcp/config gce_metadata=""
Success! Data written to: auth/gcp/config

$ vault read auth/gcp/config
Key             Value
---             -----
gce_metadata    []
iam_metadata    [project_id role service_account_id service_account_email]

$ vault write auth/gcp/config gce_metadata="default"
Success! Data written to: auth/gcp/config

$ vault read auth/gcp/config
Key             Value
---             -----
gce_metadata    [instance_creation_timestamp instance_id instance_name project_id project_number role service_account_id service_account_email zone]
iam_metadata    [project_id role service_account_id service_account_email]

$ vault write auth/gcp/config gce_metadata="role"
Success! Data written to: auth/gcp/config

$ vault read auth/gcp/config
Key             Value
---             -----
gce_metadata    [role]
iam_metadata    [project_id role service_account_id service_account_email]

$ vault write auth/gcp/config gce_metadata="default,role"
Error writing data to auth/gcp/config: Error making API request.

URL: PUT http://localhost:8200/v1/auth/gcp/config
Code: 400. Errors:

* 1 error occurred:
	* failed to parse gce metadata: ["default" "role"] contains default - default can't be used in combination with other fields


$ vault write auth/gcp/config gce_metadata="badfield"
Error writing data to auth/gcp/config: Error making API request.

URL: PUT http://localhost:8200/v1/auth/gcp/config
Code: 400. Errors:

* 1 error occurred:
	* failed to parse gce metadata: "badfield" contains an unavailable field, please select from "instance_creation_timestamp, instance_id, instance_name, project_id, project_number, role, service_account_id, service_account_email, zone"

Test output

$ make test
?   	github.com/hashicorp/vault-plugin-auth-gcp	[no test files]
ok  	github.com/hashicorp/vault-plugin-auth-gcp/plugin	(cached)
?   	github.com/hashicorp/vault-plugin-auth-gcp/plugin/cache	[no test files]

TODO

• Write docs (separate PR)

[austingebauer]

LGTM