[ubuntu-dev] allow using `supervisorctl`

[jankeromnes]

All Janitor containers use supervisor to run multiple programs in the background (e.g. an SSH daemon, the Cloud9 SDK, noVNC server, etc) and automatically restart them if they encounter a problem.

Our current configuration file is quite basic, and is missing a few fields that would enable using supervisorctl from inside the containers directly. See improvement suggestions here and here.

Additionally we could also move the configuration file from /etc/supervisord.conf to /etc/supervisor/supervisord.conf and include any files from /etc/supervisor/conf.d/*.conf (this would allow dockerfiles like the one for Discourse to simply import a new file like /etc/supervisor/conf.d/discourse.conf instead of appending rules to /etc/supervisord.conf.