Enable client address in Horizon's logs.

Enable client address in Horizon's logs.

Horizon's backends (httpd) see IP address of the haproxy in the logs instead
of the client address.
This patch allows to:
  - Install the remoteip httpd module [1].
  - Use the X-Forwarded-For HTTP header and override the haproxy address.
  - Configure the Horizon's logs with the client address via httpd logformat.

[1] https://httpd.apache.org/docs/2.4/mod/mod_remoteip.html
[2] https://httpd.apache.org/docs/2.4/mod/mod_log_config.html#logformat

Change-Id: Ib2f215913065426848b48f6293f33a75aff3d328
Depends-On: I54f0f5549d64768dacca71539c71a28cc99d9d95