[Improvement] cluster hscale policy always prefers SNAPSHOT over backup tools

[shanshanying]

1. choose some in-tree persistent disk driver that It does not support snapshots as your default sc
2. kbcli cluster create mysql mycluster
3. kbcli cluster hscale mycluster --component mysql --replicas 2
4. cluster status turns to updating
5. but Backup has failed with following msg

In our cluser controller, we always prefers snapshotDataClone over backupDataClone, controlled by the env VOLUMESNAPSHOT. It is a global env. But each cluster has its own StorageClass, and not all SC supports snaptshot.
We should leave the choice to users or at least to each cluster.