Add a "Test Config" option to the Nginx HA SSL Proxy addon

One of nginx’s important functions is the ability to verify that you have a valid configuration before actually making that configuration active.

It would be awesome if there was a way to fire off the “nginx configtest” command from within the addon settings for the Nginx SSL proxy so that before you click “Restart” you can confirm that your proxy server won’t go down due to a config error.