Enable storage replication for V2 and add backfill tool

Fixes #1501

util/registry/replication.py

@@ -0,0 +1,14 @@
+import features
+import json
+from data import model
+
+from app import image_replication_queue
+
+def queue_storage_replication(namespace, storage):
+  """ Queues replication for the given image storage under the given namespace (if enabled). """
+  if features.STORAGE_REPLICATION:
+    namespace_user = model.user.get_namespace_user(namespace)
+    image_replication_queue.put([storage.uuid], json.dumps({
+      'namespace_user_id': namespace_user.id,
+      'storage_id': storage.uuid,
+    }))