(GKE) Stop 'gcloud preview' call from failing Jenkins jobs.

cluster/gke/util.sh

@@ -244,7 +244,7 @@ function detect-minion-names {
   detect-project
   detect-node-instance-group
   MINION_NAMES=($(gcloud preview --project "${PROJECT}" instance-groups \
-    --zone "${ZONE}" instances --group "${NODE_INSTANCE_GROUP}" list \
+    --zone "${ZONE}" instances --group "${NODE_INSTANCE_GROUP}" list --quiet \
     | cut -d'/' -f11))
   echo "MINION_NAMES=${MINION_NAMES[*]}"
 }