Fix deleting repos and images under MySQL

MySQL doesn't handle constraints at the end of transactions, so deleting images currently fails. This removes the constraint and just leaves parent_id as an int

data/model/image.py

@@ -303,7 +303,7 @@ def set_image_metadata(docker_image_id, namespace_name, repository_name, created
 
     if parent:
       fetched.ancestors = '%s%s/' % (parent.ancestors, parent.id)
-      fetched.parent = parent
+      fetched.parent_id = parent.id
 
     fetched.save()
     fetched.storage.save()