DYNDNS: Drop support for legacy NSUPDATE
    
    We should drop support for legacy versions of NSUPDATE that don't
    support 'realm' option. The option 'realm' was added in
    BIND 9.8.0a1.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/2817
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>