Fix external-snapshotter sidecar container RBAC rule

[xing-yang]

We don't need "create", "delete", "update" for PV and PVC resources in the external-snapshotter RBAC rule. We only need "get", "list", "watch". We should clean up the following rbac.yaml file.

https://github.com/kubernetes-csi/external-snapshotter/blob/master/deploy/kubernetes/rbac.yaml

[wackxu]

Will fix this

[pohly]

Once this is fixed, please ensure that it propagates also to those places which use the modified file:

• add e2e test for snapshot kubernetes/kubernetes#69036
• https://github.com/kubernetes-csi/docs/blob/master/book/src/Example.md#create-rbac-rules-for-csi-snapshotter

[pohly]

If you want, file a bug against kubernetes-csi/docs and assign it to me.