Fix HAProxy config docs
Fix HAProxy config docs

The HAProxy configuration doesn't appear in openstack_user_config.yml
and the example file in the conf.d directory isn't well documented.
This documentation patch alerts deployers to the example configuration
file and tells them how to activate/configure it.

Closes-Bug: 1535874

Change-Id: I6a165f90914570f52a0205622fedf6c91e5ba3fb

BjoernT/os-ansible-deployment,yanyao/openstack-ansible,openstack/openstack-ansible,cfarquhar/openstack-ansible,bunchc/openstack-ansible,BjoernT/openstack-ansible,robb-romans/openstack-ansible,major/os-ansible-deployment,BjoernT/os-ansible-deployment,Logan2211/openstack-ansible,BjoernT/openstack-ansible,stackforge/os-ansible-deployment,yanyao/newton,robb-romans/openstack-ansible,yanyao/newton,mrda/openstack-ansible,Logan2211/openstack-ansible,major/openstack-ansible,mancdaz/openstack-ansible,mancdaz/openstack-ansible,cfarquhar/openstack-ansible,cloudnull/os-ansible-deployment,bunchc/openstack-ansible,major/os-ansible-deployment,mrda/openstack-ansible,os-cloud/os-ansible-deployment,BjoernT/os-ansible-deployment,yanyao/openstack-ansible,os-cloud/os-ansible-deployment,stackforge/os-ansible-deployment,mrda/openstack-ansible,openstack/openstack-ansible,major/openstack-ansible,weezer/openstack-ansible,cloudnull/os-ansible-deployment,weezer/openstack-ansible