[Proposal] Add a filebeat-like elasticsearch docker logging plugin

[fearful-symmetry]

The full proposal can be found here: https://docs.google.com/document/d/1gNH0SnasbcFuhoRf2ra6M8TAycD2shSIP7uKdcy78k0/edit#

The initial discuss issue is here: #10047. This is a longstanding request from the cloud team, and many competing logging solutions have docker log drivers available.

After talking extensively with @urso , I made a second proposal with his help: https://docs.google.com/document/d/1GIMbEuoHSLrl2MMVD_YF2gbZPSc5R72m2daQRTGSjz0

This proposal calls for a front end CLI utility, similar to functionbeat, that's responsible for controlling and configuring the docker plugin.

[fearful-symmetry]

Thinking more about the output/config issue. Since we still have global state, we could just spin up a new output/libbeat instance every time we get a different output config, as opposed to per-container or some hacked up global output. Something like:

configHash := hashsum(config["hostname"] + config["index"] + config["auth"])

if _, ok := libbeatHandlers[configHash]; !ok{
libbeatHandlers[configHash] = newHandler(config)
}

It's still a bit of overhead, but at least we can scale for the 90% case where everything is going to one cluster.