RFE: Support setting config-file and config-dir from environment variables

[Ben Nemec]

Just had a deployer ask about setting the config-file and config-dir options via environment variables (specifically for WSGI-based deployments). It looks to me like we don't support that today[0] because the code that processes those options doesn't use the normal lookup logic so it doesn't benefit from the environment driver. I think it would be fairly simple to add logic to look up those options from the environment though. If we do so, the env vars should match the format that the environment driver uses (e.g. OS_DEFAULT__CONFIG_FILE).

0: https://github.com/openstack/oslo.config/blob/edcb46deae997056020acff8d24bf328778624c9/oslo_config/cfg.py#L2907