Poor IPv6 support for nginx['listen_addresses']

Currently, to get gitlab listening over IPv6 I have to use a directive like this in gitlab.rb:

nginx['listen_addresses'] = ['1.2.3.4', '[2001::1.2.3.4]']

This works, right now, but it seems a bit fragile!

@dblessing noted https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-cookbooks/gitlab/templates/default/nginx-gitlab-http.conf.erb#L40-42 is where it's used.

It's trivial to detect an IPv6 address (presence of a colon, or you can use a full parse-and-check-family routine if you like), so the necessary [] characters could be programmatically added instead.