Fix ldap issues

Fixes some minor issues that have not been reported yet:

- internal userExists() respecitvely groupExists() checks, avoids strange behaviour on multi-server setups
- avoid debug message were not necessary, avoids confusion
- avoid unnecessary func call, a leftover from a former refactoring
- make sure that settings are fetched from appid = user_ldap only

reviewers please, e.g. @Raydiation @bartv2 @tanghus or others :)