Clean up Horizon Apache configuration files in clean.sh

The horizon cleanup function wasn't being called at all during
cleanup which left the Apache configuration.

Change-Id: Iff5336d0c5e79cfc82f1c648afaabb869d86020e
diff --git a/clean.sh b/clean.sh
index 0641bff..452df02 100755
--- a/clean.sh
+++ b/clean.sh
@@ -95,6 +95,7 @@
 cleanup_nova
 cleanup_neutron
 cleanup_swift
+cleanup_horizon
 
 if is_service_enabled ldap; then
     cleanup_ldap