.conf suffix for the horizon config on Fedora

On Fedora by default the *.conf imported only from
the /etc/httpd/conf.d/.

Changing the default config name to horizon.conf with all
distribution in order to have a simple logic.

Change-Id: I08c3e825f697640fd73ac1f9c569f313abc3c04f
Merge ".conf suffix for the horizon config on Fedora"