remove supervisor, SSH, & supporting stuff from base Dockerfile

According to this blog post, we shouldn't need SSH running in our containers:
http://jpetazzo.github.io/2014/06/23/docker-ssh-considered-evil/

There may still be some containers that need supervisor for other
reasons. If so, we can create a new intermediate container that installs that.

For the others, there is still work to do to move them away from
supervisor and towards running their primary commands
directly. I *think* we can just do that opportunistically as we touch
them for other reasons.

[Pivotal](https://www.pivotaltracker.com/story/show/75984728)