Fix Fedora CoreOS on GCP proposing controller recreate

* With Fedora CoreOS image stream support (#727), the latest
resolved image will change over the lifecycle of a cluster.
* Fix issue where an image diff proposed replacing a Fedora
CoreOS controller on GCP, introduced in #727 (unreleased)
* Also ignore image diffs to the GCP managed instance group
of workers. This aligns with worker AMI diffs being ignored
on AWS and similar on Azure, since workers update themselves.

Background:

* Controller nodes should strictly not be recreated by Terraform,
they are stateful (etcd) and should not be replaced
* Across cloud platforms, OS image diffs are ignored since both
Flatcar Linux and Fedora CoreOS nodes update themselves. For
workers, user-data or disk size diffs (where relevant) are allowed
to recreate workers templates/configs since these are considered
to be user-initiated declarations that a reprovision should be done

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

@@ -59,7 +59,10 @@ resource "google_compute_instance" "controllers" {
   tags           = ["${var.cluster_name}-controller"]
 
   lifecycle {
-    ignore_changes = [metadata]
+    ignore_changes = [
+      metadata,
+      boot_disk[0].initialize_params
+    ]
   }
 }
 

google-cloud/fedora-coreos/kubernetes/workers/workers.tf

@@ -64,6 +64,9 @@ resource "google_compute_instance_template" "worker" {
   }
 
   lifecycle {
+    ignore_changes = [
+      disk[0].source_image
+    ]
     # To update an Instance Template, Terraform should replace the existing resource
     create_before_destroy = true
   }