Fix the accept_status code for create_gerrit_branch

- gerrit create branch endpoint returns code 201 instead of 200

Bug:
R=agable

Change-Id: I07697e87e506d5545e9abd501775e0fc30a48f80
Reviewed-on: https://chromium-review.googlesource.com/482522Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Di Mu <dimu@chromium.org>

gerrit_util.py

@@ -776,7 +776,7 @@ def CreateGerritBranch(host, project, branch, commit):
   path = 'projects/%s/branches/%s' % (project, branch)
   body = {'revision': commit}
   conn = CreateHttpConn(host, path, reqtype='PUT', body=body)
-  response = ReadHttpJsonResponse(conn)
+  response = ReadHttpJsonResponse(conn, accept_statuses=[201])
   if response:
     return response
   raise GerritError(200, 'Unable to create gerrit branch')