Adding a running method to the Backup Model

* Adds a query classmethod to Base Model for
  more complex queries such 'in_'
* Adds a running classmethod to Backup Model
  which returns any currently running backups
  for an instance.
* Add a checksum field to the Backup Model.
* Refactor the DBBackup to reuse base methods

Change-Id: Iaf6903af88c7f4caa20921f1ca63354999ec6348
Merge "Adding a running method to the Backup Model"