rbd: return 0 and an empty list when pool is entirely empty

rbd_list will return -ENOENT when no rbd_directory object
exists. Handle this in the cli tool and interpret it as success with
an empty list.

Add this to the release notes since it changes command line behavior.

Fixes: #6693
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #1028 from ceph/wip-empty-rbd-ls

rbd: return 0 and an empty list when pool is entirely empty

Reviewed-by: Sage Weil <sage@inktank.com>