Simplify upstart + gunicorn config

Upstart:
* Move to native upstart config options instead of using gunicorn options
* chdir to current artifact before script starts (avoids a race condition
  of a deployment happening mid-restart).
* Depend on python search path being configured by deploy process

Gunicorn:
* Remove pidfile conf, since this is not needed by upstart
* Remove proc_name conf, since the python package to enable it
  is not installed.
* Rename log files.