shutdown: do final unmounting only if not running inside the container

If we run in the container then we run in a mount namespace. If namespace dies
then kernel should do unmounting for us, hence we skip unmounting in containers.

Also, it may be the case that we no longer have capability to do umount,
because we are running in the unprivileged container.

See: http://lists.freedesktop.org/archives/systemd-devel/2014-October/023536.html