ipa: Nested netgroups do not work
    
    We lowercase the keys to the hash table used to store netgroups
    but do not lowercase it when reading the table. This results
    in nested netgroups not being found when they should and
    the processing fails.
    
    The lowercasing does not seem to be necessary anymore (not
    sure if it ever was) so we can skip it.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3159
    
    Reviewed-by: Petr Cech <pcech@redhat.com>