Fallback values for $ENV in config.yaml

[pavankrish123]

Is your feature request related to a problem? Please describe.

In cases where config.yml relies on environment variables (example)

oauth2client:
    client_id: "${CLIENT_ID}"

it would be nice to have some way to specify fall back environment var if the env variable is not set instead of evaluating to blank string. Something in lines of

oauth2client:
    client_id: "${CLIENT_ID:<default-id>}"

[bogdandrutu]

This issue does not have enough requests to justify extra dependencies and logic. We should wait until more users need this.