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>
(cherry picked from commit ac547a5b7dc94282f079aef78e66348d99d9d5e9)

Conflicts:
	PendingReleaseNotes
	src/rbd.cc