Set default values for careful, warning and critical.

def set_default_cwc(self, section,
                        option_header=None,
                        cwc=['50', '70', '90']):
        """Set default values for careful, warning and critical."""
        if option_header is None:
            header = ''
        else:
            header = option_header + '_'
        self.set_default(section, header + 'careful', cwc[0])
        self.set_default(section, header + 'warning', cwc[1])
        self.set_default(section, header + 'critical', cwc[2])