Fix X-Forwarded-{Host,Server} headers for Stagecraft

Django doesn't support multiple, comma separated host names in the X-Forwarded headers.
https://code.djangoproject.com/ticket/9064

- Upgrade the nginx puppet module to bring in the ability to switch how the X-Forwarded headers are set. 
- Add a flag to `performanceplatform::gunicorn_app` to describe an app as a Django app.
- Configure Stagecraft to be a Django app.