rbd: handle --{group,image}-namespace in "rbd group image {add,rm}"

rbd: handle --{group,image}-namespace in "rbd group image {add,rm}"

Currently only passing the namespace as part of the group or image spec
works.  If --group-namespace or --image-namespace options are used, the
namespace isn't picked up.

Fixes: https://tracker.ceph.com/issues/69324
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>