Meta issue for transforming the Elasticsearch output index option into an index pattern (#921) and for cleaning up the outputs configuration around indices.

Tasks:

• In the Redis output, rename the index option to key, deprecating index for now. Renamed redis.index option to redis.key #2077.
• In the file output, remove the index option. Renamed redis.index option to redis.key #2077
• In the elasticsearch output, make index accept an index pattern Configurable Elasticsearch index pattern #2119
• Remove the index hack from the publisher code, and use the beatName as appropiate
• Update docs