etcd upgrade warning: add docs link, fixed etcd2 behavior, print non-interactive

cluster/gce/upgrade.sh

@@ -447,18 +447,20 @@ if [[ "${master_upgrade}" == "false" ]] && [[ "${node_upgrade}" == "false" ]]; t
   exit 1
 fi
 
-# prompt if etcd storage media type isn't set
-if [[ -z "${STORAGE_MEDIA_TYPE:-}" ]]; then
+# prompt if etcd storage media type isn't set unless using etcd2 when doing master upgrade
+if [[ -z "${STORAGE_MEDIA_TYPE:-}" ]] && [[ "${STORAGE_BACKEND:-}" != "etcd2" ]] && [[ "${master_upgrade}" == "true" ]]; then
+  echo "The default etcd storage media type in 1.6 has changed from application/json to application/vnd.kubernetes.protobuf."
+  echo "Documentation about the change can be found at https://kubernetes.io/docs/admin/etcd_upgrade."
+  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 "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
-    echo "The default etcd storage media type in 1.6 has changed from application/json to application/vnd.kubernetes.protobuf."
-    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 "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!!"
-    read -p "Would you like to continue? [y/N] " confirm
+    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