remove hardcoded volume class name, fix #417

from kube doc

A volume snapshot can request a particular class by specifying the name
of a VolumeSnapshotClass using the attribute volumeSnapshotClassName. If
nothing is set, then the default class is used if available.

https://kubernetes.io/docs/concepts/storage/volume-snapshots/

snapshotEngine/createVolumeSnapshot.yaml

@@ -6,6 +6,5 @@ metadata:
  labels:
  history_mode: rolling
 spec:
- volumeSnapshotClassName: test-snapclass
  source:
  persistentVolumeClaimName: ""