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/v1/models_interface.py

@@ -203,3 +203,8 @@ class DockerRegistryV1DataInterface(object):
     Returns a sorted list of repositories matching the given search term.
     """
     pass
+
+  @abstractmethod
+  def is_namespace_enabled(self, namespace_name):
+    """ Returns whether the given namespace exists and is enabled. """
+    pass