support for running linux backend remotely

This changes all usage of exec.Command to manually constructing, so that it
doesn't resolve the executable path locally. A RemoteCommandRunner is added
that wraps the command with SSH.

Added a private network to the Vagrantfile and removed the shared folders for
running Go. The Vagrant VM should now be able to just run in the background
and the Garden server itself can be run locally.
use df -P to determine depot mount point
unpend container info tests
fix real command runner
add run-garden-remote-linux helper, expand README