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

test/registry/protocols.py

@@ -50,6 +50,7 @@ class Failures(Enum):
   INVALID_IMAGES = 'invalid-images'
   UNSUPPORTED_CONTENT_TYPE = 'unsupported-content-type'
   INVALID_BLOB = 'invalid-blob'
+  NAMESPACE_DISABLED = 'namespace-disabled'
 
 
 class ProtocolOptions(object):