Issue where volumeSnapshots have 'ReadyToUse' field set to false.

Hi,

I ran into an issue when I create a VolumeSnapshotContent and a VolumeSnapshot. The volumeSnapshot never enters the 'ReadyToUse: True' state. Here is an example of the VolumeSnapshot- 

- apiVersion: snapshot.storage.k8s.io/v1alpha1
  kind: VolumeSnapshot
  metadata:
    creationTimestamp: "2020-09-01T20:15:49Z"
    finalizers:
    - snapshot.storage.kubernetes.io/volumesnapshot-protection
    generation: 2
    name: <>
    namespace: <>
    resourceVersion: <>
    selfLink: <>
    uid: <>
  spec:
    snapshotClassName:<>
    snapshotContentName: <>
    source: null
  status:
    creationTime: "2020-09-01T19:26:46Z"
    readyToUse: false
    restoreSize: 80Gi

After some debugging, I observed that the csi-snapshotter attempts to reconcile the volumesnapshot with the content periodically.

It attempts to get the underlying Snapshot status by performing a ListSnapshots operation. https://github.com/kubernetes-csi/external-snapshotter/blob/d24d2b9820079c6658e1078d59071aebb6e0d467/pkg/snapshotter/snapshotter.go#L132

This call succeeds but doesn't set the 'ReadyToUse' field. It results in default 'False' value and never marks the VolumeSnapshot as 'ReadyToUse' 

Reference to ListSnapshots->getCSISnapshot  Operation - https://github.com/dell/csi-vxflexos/blob/2facca1c4ff7256763b0b2d892b1ff2880156fd5/service/service.go#L320

An example of how others implement this- https://github.com/kubernetes-sigs/aws-ebs-csi-driver/blob/b0079ef15ef984ab550fd5bc709f6a3fd5cc1cdd/pkg/cloud/cloud.go#L628