Add in mongo backup class

Quite a dumb interim solution, but should be useful:
 - Add backup manifest that writes to whichever box the class is added to
 - Create a backup log, and write success/fail to it on backup
 - Add a sensu check that checks for the existence of 'failure' in the log
 - Add the backup class to the mongo box role
Remove class inheritance as it’s not needed
Merge pull request #268 from alphagov/mongobackups

Run backups on mongo boxes