SDAP: Make it possible to silence errors from dereference
    
    https://fedorahosted.org/sssd/ticket/2791
    
    When a modern IPA client is connected to an old (3.x) IPA server, the
    attribute dereferenced during the ID views lookup does not exist, which
    triggers an error during the dereference processing and also a confusing
    syslog message.
    
    This patch suppresses the syslog message.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>