Update Audit/Apel log forwarding for OpenSearch
Switch to using the OpenSearch output plugin, now that we have OpenSearch 2.x.
The options copied in here are from the Kolla log forwarding settings in KA, so should be fine.
A further optimisation could be to let the Fluentd output plugin know about all the OS nodes, and do it's own load balancing, rather than sending everything via HAProxy. We can consider that upstream.