lock: remove description when unlocking

This will hopefully make it easier to track down what's leaving nodes unlocked.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
lock: pass desc to lock operation; leave on unlock

Pass the desc to the lock operation.

The unlock operation now clears desc for us; no need to do it outselves.

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

Conflicts:

	teuthology/lock.py