sudo: add ldap_sudorule_object_class_attr
    
    This option will be undocumented and its main purpose is to allow
    AD provider to use objectCategory (which is indexed) instead of
    objectClass attribute (which is not indexed).
    
    Having it as an option instead of hardcoded value gives us the ability
    to switch back to objectClass if there will be any troubles.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>