qa/workunits/rbd: fix bogus grep -v asserts in test_clone()

qa/workunits/rbd: fix bogus grep -v asserts in test_clone()

The intent of "rbd ls | grep -v clone" was probably to check that an
image with the name "clone" shows up in rbd2 pool and not in rbd pool.
However, it's very far from that -- "grep -v clone" would succeed
regardless because of an image with the name "test1" in rbd pool.

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