ceph-disk: do not mount over an osd directly in /var/lib/ceph/osd/$cluster-$id

If we see a 'ready' file in the target OSD dir, do not mount our device
on top of it.

Among other things, this prevents ceph-disk activate on stray disks from
stepping on teuthology osds.

Fixes: #5445
Signed-off-by: Sage Weil <sage@inktank.com>