Have the V2 registry endpoints raise Unauthorized with the proper header when anonymous access is disabled

Before this change, we'd raise a generic 401, which was breaking containerd and cri-o.

Fixes https://jira.coreos.com/browse/QUAY-1332

test/registry/protocols.py

@@ -82,6 +82,7 @@ class ProtocolOptions(object):
     self.request_addr = None
     self.skip_blob_push_checks = False
     self.ensure_ascii = True
+    self.attempt_pull_without_token = False
 
 
 @add_metaclass(ABCMeta)