Add support for mongodump backups via config.
Fix the rev back to 26
Remove the test cron setting of backups every 2min
Garden
Add support for mongodump backups from cron.

- Adds a backup script which is copied to /var/lib/mongodb to do the actual backup work.
- On config change, it generates the backup script from the template in the charm with the path/backup copies kept config data.
- Added some readme around the config of backup with notes on limitations such as backups continuing after a service destroy.