rbd: Add snapshot mirroring support

Add support for snapshot based replication (this feature was added in Octopus in Ceph) to cinder rbd driver and ceph backup driver. Snapshot mirroring need the fast diff feature and to enable the replication per image. This feature is very similar to the existing ceph journal replication in cinder.

This would require to add some new configuration options. I propose to add `rbd_replication_mode` in the cinder driver which would have two possible values: "journal" and "snapshot" and defaults to "journal" to not break any existing configuration. On the cinder-backup driver I propose to add a boolean `backup_ceph_image_snapshot_mirroring` to be coherent with the existing `backup_ceph_image_journals`.

==> We discussed this at the cinder meeting on 7 July [0]. Consensus is that:
 (1) rbd driver option to select journaling vs snapshot mirroring is a good idea,
 (2) for rbd backup driver, journaling only when it's available, snapshot mirroring does not seem an appropriate technology for backups (so no change to rbd backup driver).

If you have an objection to (2), please come to the next Cinder weekly meeting [1] to explain to the team. Thanks!

[0] https://meetings.opendev.org/meetings/cinder/2021/cinder.2021-07-07-14.00.log.html#l-101
[1] https://etherpad.opendev.org/p/cinder-xena-meetings