Add infra-deploy-via-ssh mechanism module

Once this manifest is in place, a slave with the deploy::slave class can do something like

    ssh deploypuppet@208.74.145.200 -i /var/lib/workspace/workspace/deploy_key/deploy_key

Which will trigger the git-clone/rsync on the master. The `command=...` parameter in authorized keys replaces whatever the slave specifies as a command, so in the event of a slave being compromised, the attacker cannot use this key to execute anything else.