Use external_url's host instead of node[fqdn] in gitlab.rb

I'm using FQDNs for my servers that are more descriptive than the hostnames I host Gitlab on. For example, this server is called dev-gitlab-main-001.example.internal while my Gitlab is hosted on gitlab.example.com. My SSL certificate is requested for gitlab.example.com etc.

Of course it's a matter of taste, but I think this is the more correct way. Ruby allows for using the host part of the external_url parameter at these places, so why wouldn't we?