librbd: ignore -EOPNOTSUPP errors when retrieving image group membership

librbd: ignore -EOPNOTSUPP errors when retrieving image group membership

The Luminous release did not support adding images to a group (it only
included the bare-minimum support for creating groups). Commit f76df32666b
incorrectly dropped support for ignoring this possible failure. This
prevents Nautilus-release clients from opening images contained within
a Luminous-release cluster.

Fixes: http://tracker.ceph.com/issues/38834
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 6f29dc69a0db3b6d982c95ab9d3b9b00a7029e37)