The current systemd unit file misses 'PartOf=ceph-rbd-mirror.target',
which results in the unit not starting after reboot.
If you have ceph-rbd-mirror@rbd-mirror.ceph-rbd-mirror0, it won't start
after reboot even if enabled.
Adding 'PartOf=ceph-rbd-mirror.target' will enable
ceph-rbd-mirror.target when ceph-rbd-mirror@rbd-mirror.ceph-rbd-mirror0
gets enabled.