Work with users defined in LDAP posixGroups

Add a configuration key `LDAP_DN_ATTRIBUTE`.

This patch enables authenticating with an LDAP instance configured to use *posixGroup* LDAP groups with *memberUid*.

Configuring these parameters allows the use of such an LDAP-server:

```
ldap.config.dn: 'dn'
ldap.config.groups.member: 'memberUid'
ldap.config.groups.class: 'posixGroup'
```