Fix paths to Mongo in jenkins_worker

@feanil While cutting a new Jenkins worker AMI, I noticed a mismatch between the Mongo directories created by the Jenkins worker upstart script and the directories in the Mongo role config.

Longer term, there's probably a better way to do this.  Maybe an optional play in the mongo role for using ephemeral storage?  Short term, though, I'd like to get this in so I can update the Jenkins workers.