[csi-rbd] detach cloned images from snapshots

[sngchlko]

If rbd snapshots have child cloned images, we can't remove the snapshots because of protecting the snapshots. We can use rbd flatten for detaching cloned images from snapshots or we can consider using RBD clone v2 feature with later versions of Mimic as well.
The short-term plan would be using rbd flatten for detaching cloned images from snapshots.

[rollandf]

To Reproduce
1. Create PVC
2. Create VolumeSnapshot from PVC
3. Create PVC from VolumeSnapshot
4. Delete VolumeSnapshot
5. RBD snapshot is still present in the Ceph Server

[Madhu-1]

closing this as this will be covered in #675