Add EKS Secret envelope encryption support (#772)

This adds support for configuring EKS clusters that utilise envelope
encryption for Secrets:

- https://aws.amazon.com/about-aws/whats-new/2020/03/amazon-eks-adds-envelope-encryption-for-secrets-with-aws-kms/

- https://aws.amazon.com/blogs/containers/using-eks-encryption-provider-support-for-defense-in-depth/

examples/secrets_encryption/main.tf

@@ -0,0 +1,118 @@
+terraform {
+  required_version = ">= 0.12.0"
+}
+
+provider "aws" {
+  version = ">= 2.52.0"
+  region  = var.region
+}
+
+provider "random" {
+  version = "~> 2.1"
+}
+
+provider "local" {
+  version = "~> 1.2"
+}
+
+provider "null" {
+  version = "~> 2.1"
+}
+
+provider "template" {
+  version = "~> 2.1"
+}
+
+data "aws_eks_cluster" "cluster" {
+  name = module.eks.cluster_id
+}
+
+data "aws_eks_cluster_auth" "cluster" {
+  name = module.eks.cluster_id
+}
+
+provider "kubernetes" {
+  host                   = data.aws_eks_cluster.cluster.endpoint
+  cluster_ca_certificate = base64decode(data.aws_eks_cluster.cluster.certificate_authority.0.data)
+  token                  = data.aws_eks_cluster_auth.cluster.token
+  load_config_file       = false
+  version                = "1.10"
+}
+
+data "aws_availability_zones" "available" {
+}
+
+locals {
+  cluster_name = "test-eks-${random_string.suffix.result}"
+}
+
+resource "random_string" "suffix" {
+  length  = 8
+  special = false
+}
+
+resource "aws_kms_key" "eks" {
+  description = "EKS Secret Encryption Key"
+}
+
+module "vpc" {
+  source  = "terraform-aws-modules/vpc/aws"
+  version = "2.6.0"
+
+  name                 = "test-vpc"
+  cidr                 = "10.0.0.0/16"
+  azs                  = data.aws_availability_zones.available.names
+  private_subnets      = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24"]
+  public_subnets       = ["10.0.4.0/24", "10.0.5.0/24", "10.0.6.0/24"]
+  enable_nat_gateway   = true
+  single_nat_gateway   = true
+  enable_dns_hostnames = true
+
+  tags = {
+    "kubernetes.io/cluster/${local.cluster_name}" = "shared"
+  }
+
+  public_subnet_tags = {
+    "kubernetes.io/cluster/${local.cluster_name}" = "shared"
+    "kubernetes.io/role/elb"                      = "1"
+  }
+
+  private_subnet_tags = {
+    "kubernetes.io/cluster/${local.cluster_name}" = "shared"
+    "kubernetes.io/role/internal-elb"             = "1"
+  }
+}
+
+module "eks" {
+  source       = "../.."
+  cluster_name = local.cluster_name
+  subnets      = module.vpc.private_subnets
+
+  cluster_encryption_config = [
+    {
+      provider_key_arn = aws_kms_key.eks.arn
+      resources        = ["secrets"]
+    }
+  ]
+
+  tags = {
+    Environment = "test"
+    GithubRepo  = "terraform-aws-eks"
+    GithubOrg   = "terraform-aws-modules"
+  }
+
+  vpc_id = module.vpc.vpc_id
+
+  worker_groups = [
+    {
+      name                 = "worker-group-1"
+      instance_type        = "t2.small"
+      additional_userdata  = "echo foo bar"
+      asg_desired_capacity = 2
+    },
+  ]
+
+  map_roles    = var.map_roles
+  map_users    = var.map_users
+  map_accounts = var.map_accounts
+}