tests: improve ceph-disk.sh setup/teardown

tests: improve ceph-disk.sh setup/teardown

Address all possible failure cases, when ceph-disk.sh completes or when
it starts with leftover from a previous interrupted run. It is assumed
that ceph-disk.sh will crash at any point.

* umount all mount points that belong to ceph-disk.sh (check the
  absolute path of the directory)
* dmsetup remove all device mapper nodes found to hold a loop device
  that ceph-disks.sh no longer uses
* losetup --detach all loop devices that ceph-disks.sh no longer uses

Signed-off-by: Loic Dachary <ldachary@redhat.com>