Update default flat roster with the passed in information.
        :return:

def _update_roster(self):
        '''
        Update default flat roster with the passed in information.
        :return:
        '''
        roster_file = self._get_roster()
        if os.access(roster_file, os.W_OK):
            if self.__parsed_rosters[self.ROSTER_UPDATE_FLAG]:
                with salt.utils.files.fopen(roster_file, 'a') as roster_fp:
                    roster_fp.write('# Automatically added by "{s_user}" at {s_time}\n{hostname}:\n    host: '
                                    '{hostname}\n    user: {user}'
                                    '\n    passwd: {passwd}\n'.format(s_user=getpass.getuser(),
                                                                      s_time=datetime.datetime.utcnow().isoformat(),
                                                                      hostname=self.opts.get('tgt', ''),
                                                                      user=self.opts.get('ssh_user', ''),
                                                                      passwd=self.opts.get('ssh_passwd', '')))
                log.info('The host {0} has been added to the roster {1}'.format(self.opts.get('tgt', ''),
                                                                                roster_file))
        else:
            log.error('Unable to update roster {0}: access denied'.format(roster_file))