Enable HAProxy forwardfor option for Horizon.

Enable HAProxy forwardfor option for Horizon.

Horizon's backends (httpd) see IP address of the haproxy in the logs instead
of the client address.
Adding forwardfor option allows to add the client address to the
X-Forwarded-For HTTP header and can be replace in the logs by configured the
backend servers with this header.

Change-Id: I54f0f5549d64768dacca71539c71a28cc99d9d95