Additional nginx server blocks

Enables optional addition of external vhosts to nginx configuration for multi-site hosts.

As can be seen by the many questions/blogposts on the topic of running multiple sites from a host configured by Gitlab Omnibus, this could be a welcome addition.

I've tried other ways (including separate nginx w/ passenger), but this small patch seems the most unobtrusive and easiest to implement.

BTW, there is probably a better include path to think of.