remove listen directive bound to only ipv4

Using devstack on a  RHEL based system results in
"Listen 0.0.0.0:80" being added to the
/etc/httpd/conf/httpd.conf.
This configures Apache to only listen to port 80 on an IPv4 interface.
This makes it not possible to access Horizon via IPv6 without
re-configuring and restarting httpd.

Removing this sed leaves the default "Listen 80" from the rpm package,
 which binds to all interfaces and will allow connection to Horizon
via IPv6.

Change-Id: I9fe8cbebff0ca6a30ceeaae0f7e035c9bb828d44
diff --git a/lib/horizon b/lib/horizon
index 78cbe8b..3271e2a 100644
--- a/lib/horizon
+++ b/lib/horizon
@@ -121,9 +121,7 @@
     if is_ubuntu; then
         disable_apache_site 000-default
         sudo touch $horizon_conf
-    elif is_fedora; then
-        sudo sed '/^Listen/s/^.*$/Listen 0.0.0.0:80/' -i /etc/httpd/conf/httpd.conf
-    elif is_suse; then
+    elif is_fedora || is_suse; then
         : # nothing to do
     else
         exit_distro_not_supported "horizon apache configuration"