Upload kubeconfig to master metadata in tests.

Till a few days ago, it was possible to ssh into master and access cluster via insecure master port.
Now, the master insecure port has been disabled, we're not able to do that anymore.

This PR aims to fix that by uploading the kubeconfig to the master metadata during cluster setup in tests.

cluster/gce/config-test.sh

@@ -525,3 +525,5 @@ WINDOWS_NODE_TAINTS="${WINDOWS_NODE_TAINTS:-node.kubernetes.io/os=win1809:NoSche
 GCE_PRIVATE_CLUSTER="${KUBE_GCE_PRIVATE_CLUSTER:-false}"
 
 ETCD_LISTEN_CLIENT_IP=0.0.0.0
+
+GCE_UPLOAD_KUBCONFIG_TO_MASTER_METADATA=true

cluster/gce/util.sh

@@ -3016,6 +3016,11 @@ function check-cluster() {
 
   # ensures KUBECONFIG is set
   get-kubeconfig-basicauth
+
+  if [[ ${GCE_UPLOAD_KUBCONFIG_TO_MASTER_METADATA:-} == "true" ]]; then
+    gcloud compute instances add-metadata "${MASTER_NAME}" --zone="${ZONE}"  --metadata-from-file="kubeconfig=${KUBECONFIG}" || true
+  fi
+
   echo
   echo -e "${color_green}Kubernetes cluster is running.  The master is running at:"
   echo