Clear the stored option value (so the default will be used)

        :param opt_name: option name
        :type opt_name: str

def clear_option_value(self, opt_name):
        """ Clear the stored option value (so the default will be used)

        :param opt_name: option name
        :type opt_name: str
        """
        if not self.has_option(opt_name):
            raise ValueError("Unknow option name (%s)" % opt_name)
        self._options[opt_name].clear()