lock.py: allow --brief to stand on its own (without --list)

I use --brief all the time, and it's annoying to have to type
the useless --list at the same time.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #221 from ceph/wip-brief

lock.py: allow --brief to stand on its own (without --list)