Terraform 0.12 (#20)

* Terraform 0.12

eks-demo/eks-cluster.tf

@@ -1,15 +1,23 @@
 resource "aws_eks_cluster" "demo" {
-  name     = "${var.cluster-name}"
-  role_arn = "${aws_iam_role.demo-cluster.arn}"
+  name     = var.cluster-name
+  role_arn = aws_iam_role.demo-cluster.arn
 
   vpc_config {
-    security_group_ids = ["${aws_security_group.demo-cluster.id}"]
-    subnet_ids         = ["${module.vpc.public_subnets}"]
+    security_group_ids = [aws_security_group.demo-cluster.id]
+    # TF-UPGRADE-TODO: In Terraform v0.10 and earlier, it was sometimes necessary to
+    # force an interpolation expression to be interpreted as a list by wrapping it
+    # in an extra set of list brackets. That form was supported for compatibilty in
+    # v0.11, but is no longer supported in Terraform v0.12.
+    #
+    # If the expression in the following list itself returns a list, remove the
+    # brackets to avoid interpretation as a list of lists. If the expression
+    # returns a single list item then leave it as-is and remove this TODO comment.
+    subnet_ids = module.vpc.public_subnets
   }
 
   depends_on = [
-    "aws_iam_role_policy_attachment.demo-cluster-AmazonEKSClusterPolicy",
-    "aws_iam_role_policy_attachment.demo-cluster-AmazonEKSServicePolicy",
+    aws_iam_role_policy_attachment.demo-cluster-AmazonEKSClusterPolicy,
+    aws_iam_role_policy_attachment.demo-cluster-AmazonEKSServicePolicy,
   ]
 }