Implement automated downgrade testing.

Node version cannot be higher than the master version, so we must
switch the node version first. Also, we must use the upgrade script
from the appropriate version for GCE.

hack/verify-flags/known-flags.txt

@@ -282,6 +282,7 @@ gather-metrics-at-teardown
 gather-resource-usage
 gce-project
 gce-service-account
+gce-upgrade-script
 gce-zone
 ginkgo-flags
 gke-cluster