ceph: use wait_for_clean before scrub

Signed-off-by: Samuel Just <sam.just@inktank.com>
ceph: verify that snaps were trimmed

Signed-off-by: Samuel Just <sam.just@inktank.com>
rados: don't clean up pools, we want to scrub them!

Signed-off-by: Samuel Just <sam.just@inktank.com>