upgrade to graylog-3

[Kevin W Monroe]

Graylog 3 is needed [1] for rich Kubernetes logging (logs annotated with k8s pods/containers/labels/etc).

There are a couple issues to consider for this upgrade:
- v3 requires elasticsearch >= 5.6
- v3 default config has changed considerably (rest + web combined into single endpoint)

More details about the major changes can be found here:

http://docs.graylog.org/en/3.0/pages/upgrade/graylog-3.0.html

To avoid breaking users with incompatible config and/or ES < 5.6 in their envs, I propose creating a new track for the graylog snap (e.g. '3.0' or '3-latest') that can be used in the snap build process to deliver v3 snaps without breaking users of the latest/[risk] channels.

The 'latest' track would continue to deliver 2.x snaps until we feel confident in the process to upgrade users to a new major version.

[1] - the graylog 'beats' input had a large overhaul between v2.5 and v3 which makes the display of k8s metadata possible: https://community.graylog.org/t/kubernetes-metadata-sent-by-filebeats-but-not-shown-in-graylog/6997/9

[Junien F (axino)]

graylog 3 is available in the "3" track.