This PR allows for setting the --config cli argument via a environment variable.

The rationale behind this is in a docker environment the config is located in /data/warpgate.yaml instead of the cli default of /etc/warpgate.yaml. Meaning if you want to run any warpgate cli commands using docker exec if you do not provide the --config argument you get an error.

This simplifies docker mamagement by allowing admins to run commands like this:

docker exec my-container warpgate client-keys

Instead of:

docker exec my-container warpgate --config=/data/warpgate.yaml client-keys