mediawiki_singlenode: port apache::vhost to apache::site

* Keep the same template and get rid of some indirection (that is, rather than
  pass $install_path to apache::vhost as $docroot, just reference $install_path
  in the template, which is local to the module anyhow.
* Don't declare File['/etc/apache2/sites-enabled/wiki'] just to ensure =>
  absent it, since the directory is managed recursively by Puppet anyhow.

Change-Id: I47f8f62034b306acb010fe1f544c3faffbe8d02d
Merge "mediawiki_singlenode: port apache::vhost to apache::site" into production