test_lock_fence.sh, rbdrw.py: rbd lock/fence test

qa/workunits/rbd/test_lock_fence.sh runs using test/rbdrw.py

rbdrw.py creates an image, locks it, and runs an I/O loop;
test_lock_fence.sh runs it, waits, and then blacklists that client,
which causes rbdrw.py to get ESHUTDOWN on operations thereafter.
Currently doesn't work with rbd caching enabled.

rbd.py gets new exception type for ESHUTDOWN

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>