DYNDNS: Don't use server cmd in nsupdate by default
    
    nsupdate command `server` should not be used for the first attempt
    to udpate DNS. It should be used only in subsequent attempts after the
    first attempt failed.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2495
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>