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/outputs.tf

@@ -0,0 +1,24 @@
+output "cluster_endpoint" {
+  description = "Endpoint for EKS control plane."
+  value       = module.eks.cluster_endpoint
+}
+
+output "cluster_security_group_id" {
+  description = "Security group ids attached to the cluster control plane."
+  value       = module.eks.cluster_security_group_id
+}
+
+output "kubectl_config" {
+  description = "kubectl config as generated by the module."
+  value       = module.eks.kubeconfig
+}
+
+output "config_map_aws_auth" {
+  description = "A kubernetes configuration to authenticate to this EKS cluster."
+  value       = module.eks.config_map_aws_auth
+}
+
+output "region" {
+  description = "AWS region."
+  value       = var.region
+}