promote release to prow staging test

Change-Id: Ife54fd1f99684281b2036342e9c99736846fcafd

Author: mattcary

deploy/kubernetes/images/prow-gke-release-staging-rc-master/image.yaml

@@ -47,7 +47,7 @@ metadata:
   name: imagetag-csi-gce-driver-prow-rc
 imageTag:
   name: k8s.gcr.io/cloud-provider-gcp/gcp-compute-persistent-disk-csi-driver
-  newName: gcr.io/k8s-staging-cloud-provider-gcp/gcp-compute-persistent-disk-csi-driver
-  newTag: "v1.4.0-rc2"
+  newName: k8s.gcr.io/cloud-provider-gcp/gcp-compute-persistent-disk-csi-driver
+  newTag: "v1.4.0"
 ---