Code review comment for ~graylog-charmers/charm-graylog:feature/multi-version

Re: my previous comments regarding v2/v3 APIs: as I don't think those bits are actually being used (since the config key that exposes them isn't exposed via config.yaml yet), they can be fixed later.

My only remaining concern remains regarding the description of the web_listen_uri config value; see my previously-posted comment on config.yaml from 2019-09-03.