Add nginx stub metrics if health check enabled (#4938)

Change-Id: Iac90beef20e63fb4a539f91836231469c573f402

roles/kubernetes/node/templates/loadbalancer/nginx.conf.j2

@@ -45,6 +45,10 @@ http {
       access_log off;
       return 200;
     }
+    location /stub_status {
+      stub_status on;
+      access_log off;
+    }
   }
   {% endif %}
 }