mediawiki::sync: run sync-common unless InitialiseSettings.php exists

Bootstrap new instances by running sync-common if a local MediaWiki
deployment does not exist.

I hate this; I really do. But fixing the mediawiki::sync cluster-fuck is going
to take two months of rage-fueled rewrites, and I don't have it in me right now.

Change-Id: I5b72236d94a77c9a9b04b6fee36a16239449e046