rbd: enable ceph-rbd-mirror.target for releases <= luminous
rbd: enable ceph-rbd-mirror.target for releases <= luminous

when `ceph-rbd-mirror.target` is not enabled, the service won't start
after a reboot because there is a dependency between these two units.

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

font/ceph-ansible,fgal/ceph-ansible,font/ceph-ansible,travmi/ceph-ansible,travmi/ceph-ansible,ceph/ceph-ansible,fgal/ceph-ansible,ceph/ceph-ansible