Prepares the inventory default under ``private_data_dir`` if it's not overridden by the constructor.

def prepare_inventory(self):
        """
        Prepares the inventory default under ``private_data_dir`` if it's not overridden by the constructor.
        """
        if self.inventory is None:
            self.inventory  = os.path.join(self.private_data_dir, "inventory")