totemudpu: Implement member_set_active

Member active is used for sending "multicast" messages only to members
of ring. This reduces network load if some nodes are intentionally down.
Only regular multicast message load is reduced (messages sent by
totemudpu_mcast_noflush_send), because special messages (like hold
cancel, join message, ...) still have to be send to all members to
ensure correct behavior.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>