Update rbac rule for external-snapshotter e2e test

In the DeleteVolumeFinalizer feature in external-snapshotter,
the external-snapshotter needs to update the PVC object to
add a Finalizer if a snapshot is being created from the PVC
and delete the Finalizer after the snapshot is created.
For that reason, we need to add "update" rbac rule for the
PVC object in external-snapshot e2e test manifest file.

DeleteVolumeFinalizer PR is here. It couldn't pass e2e test
until we fix the rbac rule in e2e.
https://github.com/kubernetes-csi/external-snapshotter/pull/47

test/e2e/testing-manifests/storage-csi/external-snapshotter/rbac.yaml

@@ -25,7 +25,7 @@ rules:
   verbs: ["get", "list", "watch"]
 - apiGroups: [""]
   resources: ["persistentvolumeclaims"]
-  verbs: ["get", "list", "watch"]
+  verbs: ["get", "list", "watch", "update"]
 - apiGroups: ["storage.k8s.io"]
   resources: ["storageclasses"]
   verbs: ["get", "list", "watch"]