Update the ostree summary file and return a path to it

def update_ostree_summary(self, release):
        """Update the ostree summary file and return a path to it"""
        self.log.info('Updating the ostree summary for %s', release['name'])
        self.mock_chroot(release, release['ostree_summary'])
        return os.path.join(release['output_dir'], 'summary')