Prepare logstash for impending deprecation of _all field

We've opened an issue over on Elasticsearch about about disabling or removing the `_all` field: https://github.com/elastic/elasticsearch/issues/19784

It would be great if Logstash could come up with a plan about whether they desire this sort of functionality. If so, then changes will need to be made in the template creating a custom field and populating it with the `copy_to` parameter on the mapping. I am opening this issue up so we can link the issues and start the discussion on Logstash's end.