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
Merge "Fix Apache site config handling on Fedora"
Merge branch 'master' of github.com:openstack-dev/devstack