rdn: Cleanup of live images

When live images are aborted for some reason, they might leave mounted
directories around. 'lb clean --purge' will properly unmount them before
deleting them.

(cherry picked from commit ee460eb720f6d6511d3c6909e57a359c4e76293b)
Signed-off-by: Holger Levsen <holger@layer-acht.org>