edxapp-extra-vars: Expose extra variables for configuration of the edxapp role

Expose extra variables for configuration overload. These were things that I was missing to migrate the codecoalition.com instance.

Of specific note, `EDXAPP_ENV_EXTRA` and `EDXAPP_AUTH_EXTRA` allow to add variables to `auth.json` and `env.json` without having to overload the whole generic_env_config/generic_auth_config dictionary. This is especially useful when adding custom djangoapps or code which define extra configuration variables.