Added parameter to enable Unicorn to listen on given IP address

Why should Unicorn listen on non-localhost IP?

Because on my environment there is a bare metal server with a reverse proxy and Gitlab runs on a virtual machines.

So I have to enable Unicorn to listen on the virtual lan IP to be reached by the reverse proxy.

Having 2 reverse proxies, one on the physical an one on the vm, looks like a waste of resources to me.
I've used the same 'style' used for gitlab_unicorn_port parameter (copy and paste is my friend).
I've checked on my dev env and it takes 127.0.0.1 if no ip is set.