Discuss: validate default value for constant_keyword

Pipeline tests ignore changes around default field values: https://github.com/elastic/package-spec/pull/194 .

I assume that this is another entrypoint for data, which previously have been set only by Metricbeat or Filebeat. Now this data is set once the ingest pipeline finishes its job and just before the event is stored in Elasticsearch. In the future we have to figure out a workaround for this.

System tests don't use any expected data files. It looks like a field for improvement.

cc @andrewkroh @jsoriano .