Prevent registry operations against disabled namespaces

Allows admins to completely wall off a namespace by disabling it

Fixes https://jira.coreos.com/browse/QUAY-869

endpoints/v2/models_interface.py

@@ -270,3 +270,9 @@ class DockerRegistryV2DataInterface(object):
     Looks up all blobs with the matching digests under the given repository.
     """
     pass
+
+  @abstractmethod
+  def is_namespace_enabled(self, namespace_name):
+    """ Returns whether the given namespace is enabled. If the namespace doesn't exist,
+        returns True. """
+    pass