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>