sssctl: Case insensitive filters
    
    Lowercase the filter in case insensitive domains.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3235
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 715abb607540945cc82355e94712da7ac9746a67)