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)

digital-ocean/container-linux/kubernetes/controllers.tf

@@ -50,6 +50,12 @@ resource "digitalocean_droplet" "controllers" {
   tags = [
     "${digitalocean_tag.controllers.id}",
   ]
+
+  lifecycle {
+    ignore_changes = [
+      "user_data",
+    ]
+  }
 }
 
 # Tag to label controllers