def key_file(self):
        """Get the path to the key file containig our auth key, or None."""
        if self.auth_key:
            key_file_path = os.path.join(orchestration_mkdtemp(), 'key')
            with open(key_file_path, 'w') as fd:
                fd.write(self.auth_key)
            os.chmod(key_file_path, stat.S_IRUSR)
            return key_file_path
Get the path to the key file containig our auth key, or None.