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.
`gitlab::config::unicorn` is also used by gitlab::ci::* classes could you setup a default value here?