LDAP drivers are too opinionated about group membership schema

[rhafer]

The groupmembership lookups in the user- and groupproviders assume that the membership attribute in a group directly maps to the user's name. So this basically only works with LDAP servers implementing the RFC2307 Schema (posixGroup). And even then only when the reva uid Attribute (OpaqueID) matches the user's name.

I think we need to make this a bit more flexible by supporting

• "native" LDAP groups. I.e. groups using the groupOfNames Objectclass, where the member as listed as full DNs in the member Attribute
• Allow the use of attributes other than the loginname for the uid for example entryUUID or objectGUID (in the ActiveDirectory case) without breaking group membership lookup.

[micbar]

Related to owncloud/ocis#3150

[rhafer]

This has been fixed with: #2708