Update configuration to support easier deployment

* GOAL: Let the deployment process write one config file, and provide an easy
  env path for loading that config in
* Add `FIRETRACKER_CONFIG_PATH` env, which specifies the config file to load
* Prefer "production" settings, but add the ability to turn on debug via a
  flag in the config (other debug stuff could go in there as well)
* Remove the funky interpreter stuff from passenger_wsgi. I believe this was
  to support per-virtualenv python versions, which I don't believe we will
  be using (and could probably implement a different way)
* Config files don't need to support multiple environments, since we write
  them out specific to the environment