DDNS: execute nsupdate for single update of PTR rec
    
    nsupdate fails definitely if any of update request fails when GSSAPI is used.
    
    As tmp solution nsupdate is executed for each update.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2783
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>