gitlab: Publish connection URL

In the previous patch we setup nginx service which listens to the world
and as such gitlab service becomes to be ready to used - so publish
backend URL.

NOTE we'll need to optimise and tweak gitlab a bit further in upcoming
    patches, so it can be really used under load and with our use-cases,
    but even now it listens to http ok and generally works.

/cc @kazuhiko, @jerome