Actually use ldap_attr_group

There is a config variable ldap_attr_group which isn't used anywhere:

    stretz@slpn-nb-mss:/tmp/pfsense$ grep -R ldap_attr_group *
    usr/local/www/wizards/openvpn_wizard.inc:                       $auth['ldap_attr_group'] = $pconfig['step1']['groupattr'];
    usr/local/www/system_authservers.php:                   $pconfig['ldap_attr_group'] = $a_server[$id]['ldap_attr_group'];
    usr/local/www/system_authservers.php:                                           "ldap_attr_user ldap_attr_group ldap_attr_member ldapauthcontainers");
    usr/local/www/system_authservers.php:                   $server['ldap_attr_group'] = $pconfig['ldap_attr_group'];
    usr/local/www/system_authservers.php:                   document.getElementById("ldap_attr_group").value = "<?=$tmpldata['attr_group'];?>";
    usr/local/www/system_authservers.php:                                                           <input name="ldap_attr_group" type="text" class="formfld unknown" id="ldap_attr_group" size="20" value="<?=htmlspecialchars($pconfig['ldap_attr_group']);?>"/>

I guess it should be used in auth.inc where currently "cn" is hardcoded.

Next: Implement RFE #2869.