title: django: exclude models from migrations
body: <p>In my django application (django 1.8) I'm using two databases, one '<code>default</code>' which is MySQL, and another one which is a schemaless, read-only database.
  I've two models which are accessing this database, and I'd like to exclude these two models permanently from data and schema migrations:</p>
  
  <ul>
  <li><code>makemigrations</code> should never detect any changes, and create migrations for them</li>
  <li><code>migrate</code> should never complain about missing migrations for that app</li>
  </ul>
  
  <p>So far, I've tried different things, all without any success:</p>
  
  <ul>
  <li>used the <code>managed=False</code> Meta option on both Models</li>
  <li>added a <code>allow_migrate</code> method to my router which returns <code>False</code> for both models</li>
  </ul>
  
  <p>Does anyone have an example of how this scenario can be achieved?
  Thanks for your help!</p>