Add SSH Proxy PEP

I just realized that the user here will need privileges to run `nsenter` or `docker exec`. I'm wondering if it would be any easier to eliminate the node sshd and custom shell, and instead have the SSH proxy talk TLS/SPDY to the node/container (https://trello.com/c/yngItPzL/311-13-in-container-command-execution-for-openshift-client-containers).