mediawiki-core-code-coverage with phpclover plugin

Add in the Jenkins Clover PHP Plugin for the MediaWiki core code
coverage report. That should generate some kind of metric dashboard on
the job page.

Plugin page:
https://wiki.jenkins-ci.org/display/JENKINS/Clover+PHP+Plugin

Require JJB to have support for the cloverphp publisher which I have
submitted upstream as https://review.openstack.org/#/c/52940/

Change-Id: I358d92bcbbc48763b306ab037e88638acf84bac0