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)

google-cloud/fedora-atomic/kubernetes/controllers.tf

@@ -57,6 +57,12 @@ resource "google_compute_instance" "controllers" {
 
   can_ip_forward = true
   tags           = ["${var.cluster_name}-controller"]
+
+  lifecycle {
+    ignore_changes = [
+      "metadata",
+    ]
+  }
 }
 
 # Controller Cloud-Init