Fix Flatcar Linux nodes on Google Cloud not ignoring image changes

* Add `boot_disk[0].initialize_params` to the ignored fields for the
controller nodes
* Nodes will auto-update, Terraform should not attempt to delete and
recreate nodes (especially controllers!). Lack of this ignore causes
Terraform to propose deleting controller nodes when Flatcar Linux
releases new images
* Matches the configuration on Typhoon Fedora CoreOS (which does not
have the issue)

google-cloud/flatcar-linux/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
+    ]
   }
 }