logstash: Add normalized_message field to all events

Copy the message of all events destined for storage in Elasticsearch
into a "normalized_message" field that is truncated to 255 characters.
This can be used in dashboards as a term search to correlate common
messages.

Change-Id: I01c50456cf0e334075acacbe2aebe5d8fc941d31
Merge "logstash: Add normalized_message field to all events" into production