Make resize tests work on GKE

(cherry picked from commit abc4e53648)

cluster/gke/util.sh

@@ -249,6 +249,21 @@ function detect-minion-names {
   echo "MINION_NAMES=${MINION_NAMES[*]}"
 }
 
+# Detect instance group name generated by gke
+#
+# Assumed vars:
+#   GCLOUD
+#   PROJECT
+#   ZONE
+#   CLUSTER_NAME
+# Vars set:
+#   NODE_INSTANCE_GROUP
+function detect-node-instance-group {
+  NODE_INSTANCE_GROUP=$("${GCLOUD}" alpha container clusters describe \
+    --project="${PROJECT}" --zone="${ZONE}" "${CLUSTER_NAME}" \
+    | grep instanceGroupManagers | cut -d '/' -f 11)
+}
+
 # SSH to a node by name ($1) and run a command ($2).
 #
 # Assumed vars: