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/test/test_v2auth.py

@@ -54,6 +54,11 @@ from test.fixtures import *
   ('repository:somenamespace/unknownrepo:pull,push', 'devtable', 'password', 200,
    ['somenamespace/unknownrepo:']),
 
+  # Disabled namespace.
+  (['repository:devtable/simple:pull,push', 'repository:disabled/complex:pull'],
+    'devtable', 'password', 400,
+   []),
+
   # Multiple scopes.
   (['repository:devtable/simple:pull,push', 'repository:devtable/complex:pull'],
     'devtable', 'password', 200,