On Ysul, some devserver services used directories already provisioned by other
roles like saas-mediawiki for /var/dataroot/<wiki domain>.
Yet, the mediawiki.dereckson.be instance is NOT hosted on the MediaWiki SaaS
code as it's a development instance to test changes and review commits for
MediaWiki core, skins and extensions. As such, it's handled as a standalone
service. It just need a folder to store data, outside home directory.
Ref T1536