hg-inner-http: expose Gunicorn options in general configuration

In !5 (merged) we just kept the configurability by the GUNICORN_CMD_ARGS environment variable that Heptapod Docker was formerly using.

I'm not 100% sure it will work, i.e., if the environment variable would still be passed down from the Docker container definition (e.g, docker-compose.yml and other mechanisms).

In any case, it'd be better to support configuring those parameters through /etc/gitlab/gitlab.rb.