Change the default value of an option
        
        :param opt_name: option name
        :type opt_name: str
        
        :param value: new default option value

def change_option_default(self, opt_name, default_val):
        """ Change the default value of an option
        
        :param opt_name: option name
        :type opt_name: str
        
        :param value: new default option value
        """
        if not self.has_option(opt_name):
            raise ValueError("Unknow option name (%s)" % opt_name)
        self._options[opt_name].default = default_val