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.

The table below summarizes what should happen on each distribution:
+--------------+---------------+--------------------------+--------------------------+
| Distro       | Filename      | Site enabling command    | Site disabling command   |
+--------------+---------------+--------------------------+--------------------------+
| Ubuntu 12.04 | site          | a2ensite site            | a2dissite site           |
| Ubuntu 14.04 | site.conf     | a2ensite site            | a2dissite site           |
| Fedora       | site.disabled | mv site.conf{.disabled,} | mv site.conf{,.disabled} |
+--------------+---------------+--------------------------+--------------------------+

Change-Id: Ia2ba3cb7caccb6e9b65380f9d51d9d21180b894e
Closes-bug: #1313765