Check Config Files for Allowed Values

Context

Was not seeing my remote environment variables because my config value was "remote_evn". Mistake on my part, but it would be nice if zappa checked config files to ensure that values being set were even values used by zappa.

Expected Behavior

When a user gives a config value not used by Zappa, it tells the user and suggests that it might be a typo. Just lowers the learning curve slightly and improves QOL.

Actual Behavior

Users are allowed to wallow in their mistakes

Possible Fix

Keep a white list of config keys and test against those.