Add logrotation for /var/log/mediawiki/*

* Declare /var/log/mediawiki in init.pp rather than sync.pp.
* Provision a specific logrotate file for /var/log/mediawiki/jobrunner.log.
* Configure jobrunner to log to /var/log/mediawiki/jobrunner.log by default.

Change-Id: I8e05cacdce0e75a4e281bf691c6937d36a6969f4