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
nginx-proxy/vhost-template.conf: Set X-Forwarded-Host

Since v1.5, Django cares about the requested Host (or
X-Forwarded-Host) [1], so we need to pass that information through.
With this Nginx config, you can set USE_X_FORWARDED_HOST [2] and parse
the appropriate hostname instead of your Docker container's IP.

[1]: https://docs.djangoproject.com/en/dev/ref/settings/#allowed-hosts
[2]: https://docs.djangoproject.com/en/dev/ref/settings/#std:setting-USE_X_FORWARDED_HOST