Fix Apache site config handling on Fedora

Allow enable/disable_apache_sites() on Fedora to gracefully fail if the
config is not present.  This is primarily an issue when the config from
a previous run is not completely cleaned out (unstack.sh vs clean.sh).

Make APACHE_CONFIG_DIR fully qualified and overrideable in local.conf.

Also fix Horizon's handling of its Apache config file to be removed
in clean.sh.

Change-Id: I78a5de579dd3b02fa2e4e7e00ac0aabe71b531ad
diff --git a/lib/horizon b/lib/horizon
index 632c7ab..02715ce 100644
--- a/lib/horizon
+++ b/lib/horizon
@@ -75,6 +75,9 @@
             sudo rm /usr/bin/node
         fi
     fi
+
+    local horizon_conf=$(apache_site_config_for horizon)
+    sudo rm -f $horizon_conf
 }
 
 # configure_horizon() - Set config files, create data dirs, etc