Move GCP proxy code to the old [] proxy's location

1. Moved code for the GCP proxy to where the [] proxy code used to live.
3. Corrected reference to the GCP proxy location.
4. Misc changes to make ErrorProne and various tools happy.

+diekmann to LGTM terraform whitelist change.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213630560

java/google/registry/proxy/terraform/modules/gke/output.tf

@@ -0,0 +1,3 @@
+output "proxy_instance_group" {
+  value = "${google_container_cluster.proxy_cluster.instance_group_urls[0]}"
+}