Allow a custom AMI to be specified as a default (#115)

* fallback to the default workers group ami_id before the official one

* add CHANGELOG

Author: erks

CHANGELOG.md

@@ -16,6 +16,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 
 - A subtle but thoughtful change. (Boomshakalaka, @self 🏀)
+- allow a custom AMI to be specified as a default (by @erks)
 - **Breaking change** Removed support for `eks-worker-*` AMI. The cluster specifying a custom AMI based off of `eks-worker-*` AMI will have to rebuild the AMI from `amazon-eks-node-*`.  (by @erks)
 - **Breaking change** Removed `kubelet_node_labels` worker group option in favor of `kubelet_extra_args`. (by @erks)
 

workers.tf

@@ -26,7 +26,7 @@ resource "aws_launch_configuration" "workers" {
   associate_public_ip_address = "${lookup(var.worker_groups[count.index], "public_ip", lookup(local.workers_group_defaults, "public_ip"))}"
   security_groups             = ["${local.worker_security_group_id}"]
   iam_instance_profile        = "${aws_iam_instance_profile.workers.id}"
-  image_id                    = "${lookup(var.worker_groups[count.index], "ami_id", data.aws_ami.eks_worker.id)}"
+  image_id                    = "${lookup(var.worker_groups[count.index], "ami_id", lookup(local.workers_group_defaults, "ami_id", data.aws_ami.eks_worker.id))}"
   instance_type               = "${lookup(var.worker_groups[count.index], "instance_type", lookup(local.workers_group_defaults, "instance_type"))}"
   key_name                    = "${lookup(var.worker_groups[count.index], "key_name", lookup(local.workers_group_defaults, "key_name"))}"
   user_data_base64            = "${base64encode(element(data.template_file.userdata.*.rendered, count.index))}"