Setup logging via ini-file from logging_conf_file option.

def _conf(cls, opts):
        """Setup logging via ini-file from logging_conf_file option."""
        logging_conf = cls.config.get('core', 'logging_conf_file', None)
        if logging_conf is None:
            return False

        if not os.path.exists(logging_conf):
            # FileNotFoundError added only in Python 3.3
            # https://docs.python.org/3/whatsnew/3.3.html#pep-3151-reworking-the-os-and-io-exception-hierarchy
            raise OSError("Error: Unable to locate specified logging configuration file!")

        logging.config.fileConfig(logging_conf)
        return True