test/cli-integration/rbd: add unmap test

test/cli-integration/rbd: add unmap test

Add a test to exercise rbd unmap.  This was inspired by and is mostly
for rbd unmap <image-name> (aka spec), but rbd unmap <device> is tested
as well.  Any significant change to krbd.cc or corresponding rbd cli
should be tested in the following three configurations at least:

- kernel < 3.14
- kernel >= 3.14, rbd.single_major=N
- kernel >= 3.14, rbd.single_major=Y

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>