fluentd: add modern parsing config

To satisfy the needs of the modern parser plugin.

Needed-By: https://review.opendev.org/c/openstack/kolla/+/823071


Co-Authored-By: Dr. Jens Harbott <harbott@osism.tech>
Change-Id: I2b748d0544c14bebefe8c62aa5aafaaa5371ce53