Ignore controller user_data changes to allow plugin updates

* Updating the `terraform-provider-ct` plugin is known to produce
a `user_data` diff in all pre-existing clusters. Applying the
diff to pre-existing cluster destroys controller nodes
* Ignore changes to controller `user_data`. Once all managed
clusters use a release containing this change, it is possible
to update the `terraform-provider-ct` plugin (worker `user_data`
will still be modified)
* Changing the module `ref` for an existing cluster and
re-applying is still NOT supported (although this PR
would protect controllers from being destroyed)

aws/container-linux/kubernetes/controllers.tf

@@ -39,7 +39,10 @@ resource "aws_instance" "controllers" {
   vpc_security_group_ids      = ["${aws_security_group.controller.id}"]
 
   lifecycle {
-    ignore_changes = ["ami"]
+    ignore_changes = [
+      "ami",
+      "user_data",
+    ]
   }
 }