mediawiki: preliminary steps to support multiple wikis

note: incomplete yet, more work still needed

with this change we just make sure that the configuration of the main wiki
(intranet.softwareheritage.org) does not get in the way of *manually* adding
other vhosts to the main dispatcher configuration file (LocalSettings.php.erb).

Proper puppetization of this will require supporting the definition of multiple
wikis in hiera configuration data