Use first server name for non-www redirects to prevent issues with naming of vhosts within defined types.

If I were to define a type named `reverse_proxy_vhost` and name the nginx vhosts inside `reverse_proxy_${name}` while providing custom values for `server_name` and setting `rewrite_www_to_non_www` to true, it would result in something like `reverse_proxy_mydomain.net` as the server name.

This PR resolves that.