Config improvements

[apostolosfilippas]

Here is the new logic:

CONFIG VARS

• Config is used only for "true" config vars -- such as EDSL_MAX_BACKOFF_SEC and so on.
• In your code, retrieve config vars using `CONFIG.get("ENV_VAR_NAME")

There is a special config var, EDSL_RUN_MODE

• If the user has not set EDSL_RUN_MODE, then we set it to "production"
• If EDSL_RUN_MODE=="production" then all Config env vars are set to their defaults
• If EDSL_RUN_MODE!="production" then we require the user to set all config env vars (in their .env, pytest.ini, or in the script)

API KEYS

• In your code, retrieve api keys using os.getenv()

[johnjosephhorton]

@apostolosfilippas should I review?