Fix Docker Auth and our V2 registry paths to support library (i.e. namespace-less) repositories.

This support is placed behind a feature flag.

test/test_v2_endpoint_security.py

@@ -3,7 +3,6 @@ import endpoints.decorated
 import json
 
 from app import app
-from util.names import parse_namespace_repository
 from initdb import setup_database_for_testing, finished_database_for_testing
 from specs import build_v2_index_specs
 from endpoints.v2 import v2_bp