Fix the way Apache site configuration files 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. a2ensite and a2dissite ignore the .conf suffix used as parameter. The default sites'
files are 000-default.conf and default-ssl.conf.

On Ubuntu 12.04, the site configuration file may have any format, as long as it is in
/etc/apache2/sites-available/. a2ensite and a2dissite need the entire file name to work. The default
sites' files are default and default-ssl.

On Fedora, any file in /etc/httpd/conf.d/ whose name ends with .conf is enabled.

On RHEL and CentOS, things should hopefully work as in Fedora.

This change puts all distribution-related site configuration file name differences in lib/apache and
the other services gets the file name 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:
+----------------------+--------------------+--------------------------+--------------------------+
| Distribution         | File name          | Site enabling command    | Site disabling command   |
+----------------------+--------------------+--------------------------+--------------------------+
| Ubuntu 12.04         | site               | a2ensite site            | a2dissite site           |
| Ubuntu 14.04         | site.conf          | a2ensite site            | a2dissite site           |
| Fedora, RHEL, CentOS | site.conf.disabled | mv site.conf{.disabled,} | mv site.conf{,.disabled} |
+----------------------+--------------------+--------------------------+--------------------------+

Change-Id: Ia2ba3cb7caccb6e9b65380f9d51d9d21180b894e
Closes-bug: #1313765
Merge "Fix the way Apache site configuration files are used, to improve OS portability"
Merge branch 'master' of github.com:openstack-dev/devstack