IPA: properly handle mixed-case trusted domains
    
    In the SSSD cache domain names are handled case-sensitive. As a result
    fully-qualified names in RDN contain the domain part in the original
    spelling. When IPA client lookup up group-memberships on the IPA server
    via the extdom plugin the names returned are all lower case. To make
    sure new DNs are generated correctly the domain part must adjusted.
    
    Related to https://fedorahosted.org/sssd/ticket/2159
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>