transition configuration to env variables

please review @danrschlosser 
So I think I figured out what the best way to do this is.  Because this up to  `DEFAULT_EVENT_IMAGE` are ADI specific (they're used by the client part of the app, not eventum), could / should we rename this file `eventum_config.py` and make a new file called `client_config.py` that stores any constants that are only pertinent to the client implementation?

Everything above these three keys is needed to run eventum.  Thoughts?