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

software/gitlab/instance-gitlab.cfg.in

@@ -7,6 +7,7 @@
 extends = {{ gitlab_parameters_cfg }}
 parts =
     directory
+    publish-instance-info
 
 #   gitlab-<prog>
 # ? mailroom
@@ -77,6 +78,11 @@ url     = ${external-url:scheme}://[${:host}]:${:port}
 user    = {{ pwd.getpwuid(os.getuid())[0] }}
 
 
+[publish-instance-info]
+recipe  = slapos.cookbook:publish
+backend_url = ${backend-info:url}
+
+
 
 #############################
 #   GitLab instance setup   #