Reset defaults.

def reset_defaults(self, config_file):
        """Reset defaults.
        """
        if not exists(config_file):
            err_exit('No such file: %(config_file)s' % locals())
        if not isfile(config_file):
            err_exit('Not a file: %(config_file)s' % locals())
        if not os.access(config_file, os.R_OK):
            err_exit('File cannot be read: %(config_file)s' % locals())
        self.set_defaults(config_file)