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

Overall, looks pretty nice.

I've provided a few comments for review. Note that only some of these "require" changes I think, but please look at all 4.

Additionally... In Graylog 3, a few APIs also changed, and I don't see those reflected in this change set. Specifically, I'm referring to these: https://git.launchpad.net/~vultaire/graylog-charm/diff/lib/charms/layer/graylog/logextract.py?h=graylog3&id=0b6162815e80e3b8ba73542b2d9dd326f74fa307

(Note the above is from a quick-and-dirty, backwards-incompatible fork of the Graylog charm I wrote for testing Graylog 3; it cannot be used as is since we need to call the correct API based on the graylog version.)