in storage media upgrade prompt, provide config for using protobuf

cluster/gce/upgrade.sh

@@ -454,17 +454,20 @@ if [[ -z "${STORAGE_MEDIA_TYPE:-}" ]] && [[ "${STORAGE_BACKEND:-}" != "etcd2" ]]
   echo ""
   echo "ETCD2 DOES NOT SUPPORT PROTOBUF: If you wish to have to ability to downgrade to etcd2 later application/json must be used."
   echo ""
+  echo "It's HIGHLY recommended that etcd be backed up before this step!!"
+  echo ""
   echo "To enable using json, before running this script set:"
   echo "export STORAGE_MEDIA_TYPE=application/json"
   echo ""
-  echo "It's HIGHLY recommended that etcd be backed up before this step!!"
-  echo ""
   if [ -t 0 ] && [ -t 1 ]; then
     read -p "Would you like to continue with the new default, and lose the ability to downgrade to etcd2? [y/N] " confirm
     if [[ "${confirm}" != "y" ]]; then
       exit 1
     fi
   else
+    echo "To enable using protobuf, before running this script set:"
+    echo "export STORAGE_MEDIA_TYPE=application/vnd.kubernetes.protobuf"
+    echo ""
     echo "STORAGE_MEDIA_TYPE must be specified when run non-interactively." >&2
     exit 1
   fi