logstash: Reduce filter worker count to 1

When running with multiple worker threads the multiline filter
occasionally becomes confused and merges multiple events together. This
can result in events that cannot be serialized for output to
Elasticsearch. Once this happens no events will be added to
Elasticsearch until the logstash service is restarted.

Change-Id: Iac2e0980bfce66f546b352a5350614bdd482160c