Fix and templatize the logic for external JWT AuthN and registry v2 Auth.

Make it explicit that the registry-v2 stuff is not ready for prime time.

endpoints/v2/blob.py

@@ -1,3 +1,6 @@
+# XXX This code is not yet ready to be run in production, and should remain disabled until such
+# XXX time as this notice is removed.
+
 import logging
 
 from flask import make_response, url_for, request
@@ -14,8 +17,11 @@ from util.http import abort
 logger = logging.getLogger(__name__)
 
 
-@v2_bp.route('/<namespace>/<repo_name>/blobs/<regex("' + digest_tools.DIGEST_PATTERN + '"):digest>',
-             methods=['HEAD'])
+BASE_BLOB_ROUTE = '/<namespace>/<repo_name>/blobs/<regex("{0}"):digest>'
+BLOB_DIGEST_ROUTE = BASE_BLOB_ROUTE.format(digest_tools.DIGEST_PATTERN)
+
+
+@v2_bp.route(BLOB_DIGEST_ROUTE, methods=['HEAD'])
 @process_jwt_auth
 @require_repo_read
 @anon_protect
@@ -29,12 +35,12 @@ def check_blob_existence(namespace, repo_name, digest):
     abort(404)
 
 
-@v2_bp.route('/<namespace>/<repo_name>/blobs/<regex("' + digest_tools.DIGEST_PATTERN + '"):digest>',
-             methods=['GET'])
+@v2_bp.route(BLOB_DIGEST_ROUTE, methods=['GET'])
 @process_jwt_auth
 @require_repo_read
 @anon_protect
 def download_blob(namespace, repo_name, digest):
+  # TODO Implement this
   return make_response('')