Need to set VolumeContentSource if creating volume from snapshot

[xing-yang]

In CreateVolume, we should set VolumeContentSource after creating volume from snapshot is successful.

return &csi.CreateVolumeResponse{
	Volume: &csi.Volume{
		VolumeId:      volumeID,
		CapacityBytes: req.GetCapacityRange().GetRequiredBytes(),
		VolumeContext: req.GetParameters(),
	},