locker: try to make up for apache timeouts

If the lock request succeeds in updating the db, but the client gets a
timeout from apache, they can now try again and get back the machines
they just locked.

Only automatic runs have a description set when locking several
machines, so this does not affect users of teuthology-lock
--lock-many, where no description can be set in the same request.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>