Do not stop hosts' SSH + mdadm services in cleanup procedure

The init script uses start-stop-daemon with the pidfile
/var/run/sshd.pid. We can't just stop the service from
outside the chroot because this terminates the process of the
host system instead of the process *inside* the chroot.

Thanks: Sebastian Pipping for debugging and bug report
Fixes #63 (GH-37, PR 71)