removes use of pconfig, purely environment

[JamesKunstle]

fixes #253 previously, pconfig was an object passed to celery workers that contained database access credentials in lieu of sending a global-scope instantiated AugurManager object.

this is replaced by created a new AugurManager object in each Celery worker process new from the available environment variables.

This should be more secure because we're not passing access parameters, in plaintext, between processes, as worker parameters.

[JamesKunstle]

Also deletes a lot of useless environment variable handling code in app.py that should have been dropped a long time ago.

Future task is to create an object that can handle AugurManager exception raising in a single line, removing the boilerplate code shared between all of the celery processes.

[JamesKunstle]

@cdolfi cool, merged.