title: Can't create user on MongoDB with PyMongo
body: <p>I have installed MongoDB 2.6.10 on Ubuntu 14.04.2 and pymongo 2.6.3.</p>
  
  <p>I can't create simple user for database other, than admin:</p>
  
  <pre><code>from pymongo import MongoClient
  
  client = MongoClient('127.0.0.1', 27017)
  client.admin.authenticate('siteRootAdmin','passw0rd')
  db = client['test']
  db.add_user('testAdmin1','passw0rd')
  
  OperationFailure: not authorized for insert on test.system.users
  </code></pre>
  
  <p>Before it I created user siteRootAdmin on admin db with 'root' role. From mongo shell I can add user to test db, but from pymongo have this error.</p>