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