Add possibility do delete old snapshot with ceph storage

[jsfrerot]

Description Be able to delete old snapshots with ceph storage.

Use case When reverting to a previous snapshot, a tree of snapshot is created and then we can't delete other snapshot as opennebula create a child/parent dependency. Ceph is able to flatten snapshots but opennebula doesn't handle it.

Interface Changes N/A, just be able to delete snapshot in a tree of snapshots.

Additional Context Original opennebula ticket: https://support.opennebula.pro/hc/en-us/requests/21067

Here is the procedure I do, that will generate a snapshot tree.

• Create a new VM
• take a first snapshot (snap1)
• poweroff the VM
• revert to snap1
• take a second snapshot (snap2)
• revert to snap2
• try to delete snap1 but it fails. and you have a tree of snapshot with parent/child structure

Progress Status

• [ ] Branch created
• [ ] Code committed to development branch
• [ ] Testing - QA
• [ ] Documentation
• [ ] Release notes - resolved issues, compatibility, known issues
• [ ] Code committed to upstream release/hotfix branches
• [ ] Documentation committed to upstream release/hotfix branches