Remove distinction between logstash agent and server

[martinb3]

When there's only one recipe, we'd stop seeing logstash installed twice on every server. There would also no longer be a race condition between when elasticsearch is installed and when logstash decides if it's an agent or a server instance.

[martinb3]

I think this seems like a bad idea now. Converting from one instance to the other could be unpleasant (e.g. an ELK server that is also sending logs to other elk servers).