map-unmap.sh: drop the get_id() logic

Take advantage of the fact that 'rbd map' will now talk to udev and
output the device that got assigned by the kernel to the newly created
mapping.  Drop the get_id() cruft, udevadm settle and chown calls.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
map-unmap.sh: fail if 'rbd rm' fails

Fail if 'rbd rm' fails - most probably it'd fail with "image still has
watchers" and in that case it's a bug in the kernel client which we do
want to notice.  Also nuke the trap-based error handling - cleanup() is
half-baked and not really necessary here.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #2041 from ceph/wip-map-unmap

Wip map unmap

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>