Disambiguous between config using event.sprintf and static ones

Here is an idea following a discussion on logstash-user groups where someone tried to dynamicly set the host parameter of elasticsearch_http output from event field, 

This proposal tag the config items that can (or cannot) use _dynamic value_ aka sprintf under the hood.
Example done on the elasticsearch_http, we can do the full scan afterwards...

HTH

PS: I silly dreamt that reaching the 200th Pull Request would trigger the next merge party on your side, pretty please... ;°)