Adds a flag to specify root working directory.

--root_dir specifies the directory kubelet will use for it's
procedures. Currently used for volume mounts.
Instead of Stat, recommend just calling MkdirAll, and whatever you do, I think it's important to check the error on os.MkdirAll; do not want that to fail silently. In fact, I think that should fail in a way that causes us to not attempt to start the pod. Otherwise, users are going to have a very unpleasant debugging experience. So maybe I'm asking for SetUp() to return an error.