Resolve "Make the mapping between Kerberos and LDAP configurable"

What does this MR do?

This merge request introduces two new settings two make the mapping between Kerberos and LDAP more configurable as wished in #2906 This MR adds the possibility to enable simple_ldap_linking. If simple ldap linking is activated and someone tries to authenticate via Kerberos only two test have to succeed in order to successfully authenticate the user. Firstly the Kerberos username have to match ldap uid and secondly the Kerberos realm has to be listed in the simple_ldap_linking_allowed_realms setting.

What are the relevant issue numbers?

#2906

Does this MR meet the acceptance criteria?

Edited by Christopher Schenk