Fix the way Apache configuration filenames are used, to improve OS portability

On Ubuntu 14.04, the site configuration file must have a .conf suffix for a2ensite and a2dissite to
recognise it. The way it was implemented worked on Fedora and Ubuntu 12.04, however.

This change puts all distribution-related site configuration filename differences in lib/apache and
the other services gets the filename for its sites using the new exported function
apache_site_config_for <sitename>.

It also makes Fedora disabled sites use the .conf.disabled suffix instead of removing the .conf from
the file name.

Change-Id: Ia2ba3cb7caccb6e9b65380f9d51d9d21180b894e
Closes-bug: #1313765