Adding subnets taggings capability when creating a VPC

[tminussi]

Motivation

When creating Kubernetes (k8s) clusters in EKS, subnets tagging is mandatory. Currently, it's not possible to create such clusters using this VPC module.

Changes

This PR aims at enabling the tagging capability on subnets created by the VPC module

Usage

In order to tag the subnets, simply inject values for public_subnet_tags and private_subnet_tags

  public_subnet_tags = {
    "kubernetes.io/cluster/cluster-name" = "shared"
    "kubernetes.io/role/elb"                      = "1"
  }

  private_subnet_tags = {
    "kubernetes.io/cluster/cluster-name" = "shared"
    "kubernetes.io/role/internal-elb"             = "1"
  }

[mpas]

@tminussi Looks ok, but can you make a small note in the documentation to indicate why we have a separate public_subnet_tags and private_subnet_tags?

[tminussi]

@mpas I have update both the README.md file as well as the examples under examples/