Fix mongo on dev vm

 - Dont try and mount a non existent disk
 - Move relationship requiring the mount before mongo to the mount
   itself so we dont require something that doesnt exist
 - Add a hacky extra parameter to require the logshipper config - need
   to do this because the logshipper stuff is defined in hiera, which
   doesnt let us set relationship information, and we cant setup
   logshipper until the mongo log file exists. Cant setup logshipper on
   the dev vm as there is no logstash instance running.
Merge pull request #614 from alphagov/fix-mongo-on-dev-vm

Fix mongo on dev vm