LDAP setup use an OpenLDAP specific syntax

FD is asking for "+" as attributes which does not seem to work for 389DS. It’s useless anyway as all we want is namingContexts attribute.

(from redmine: issue id 5483, created on 2017-04-20, closed on 2017-04-24)

Fixes #5483 Removed uses of "+" as an attribute asked in ldapsearch as it seems openldap specific

Also removed get_root_dse unused method