Improve 503 page

[iamtew]

When adding a new gateway VGA HAproxy is reconfigured and reloaded before the upstream server is fully responding, and in that case will show the user a HTTP 503 Service Unavailable.

As 503 is a temporary error, and having a proper maintenance page for it will prevent search engines from delisting websites. See:

• https://webmasters.googleblog.com/2011/01/how-to-deal-with-planned-site-downtime.html
• http://serverfault.com/questions/487201/haproxy-putting-up-a-maintenance-page
• https://rimuhosting.com/knowledgebase/creating-a-maintenance-page-for-your-site

[olafmol]

For proper canary releasing expected behaviour visitors should see the former version and no 503s, until the newly added version is up and running correctly (healthy) and then traffic can be routed with the conditions and weight settings as defined.