cube-plugin deletes ALL disk only snapshots

[deajan]

When making a disk only snapshot then running cube backup, cube will flatten ALL snapshots in order to remove it's own.
While this behavior makes sense, we should have a checkbox which will determine if we delete all or only cube possible snasphots for KVM vms.

We also need to handle metadata deleting

[deajan]

We need to extract the bottom base image for CUBE and pass it with --base to blockcommit op.