Delete the instance
        
        Args:
            instance (AtlasServiceInstance.Instance): an existing instance
            
        Returns:
            DeprovisionServiceSpec: Status

def delete(self, instance):
        """Delete the instance
        
        Args:
            instance (AtlasServiceInstance.Instance): an existing instance
            
        Returns:
            DeprovisionServiceSpec: Status
        """
        
        #TODO: Really drop the database based on a policy set in `instance.parameters`.
        #
        #      We need :
        #      - Set a policy in parameters of the instance (eg: policy-on-delete : retain|drop    => default to retain)
        #      - to check that the database name `instance.get_dbname()` is not in use by another instance (shared database)
        #      - credential on the Atlas cluster `instance.get_cluster()` to drop the database
        #
        
        self.backend.storage.remove(instance)
        
        return DeprovisionServiceSpec(False, "done")