nginx-proxy/vhost-template.conf: Use $proxy_add_x_forwarded_for

nginx-proxy/vhost-template.conf: Use $proxy_add_x_forwarded_for

Instead of $remote_addr.  This gives a chain of remote addresses if we
have multiple layers of proxies [1]:

  $proxy_add_x_forwarded_for

  Contains client request-header "X-Forwarded-For" with separated by
  comma $remote_addr. If there is no X-Forwarded-For request-header,
  than $proxy_add_x_forwarded_for is equal to $remote_addr.

[1]: http://wiki.nginx.org/HttpProxyModule#.24proxy_add_x_forwarded_for