rbd: add a test helper to get a conn for a given cluster by config
rbd: add a test helper to get a conn for a given cluster by config

This function can be used to connect to a non-default ceph cluster,
in a simple, convenient way similar to the function we have for
connecting to the default cluster.

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph