Run backups on mongo boxes

Provides a totally chilled out experience via nightly backups
![](http://25.media.tumblr.com/bfec814bba9b14851fe99d1400d03192/tumblr_n19akhEB3R1ql2603o1_500.jpg)
- Currently backs up nightly to the same box mongo is running on
- Writes success/fail line to a log after backing up
- Adds a sensu check to ensure that a backup has run, tailing the log

*NB:* This is an interim step before getting a proper backups box as described in https://www.pivotaltracker.com/story/show/65578342 and https://www.pivotaltracker.com/story/show/66160874