puppetize properly mediawiki vhost support

In rSPPROF2541f5e18942 I've just made sure that the current puppetization of our main wiki (intranet.softwareheritage.org) doesn't get in the way of //manually// adding other wikis as different virtual hosts. But that is no proper puppetization of support for multiple wikis hosted on the same server. We should do that. (See TODO/XXX comments in manifests/mediawiki.pp)

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Migrated from T352 (view on Phabricator)