Add tests for KafkaAdminClient

[jeffwidman]

I need to add tests for the KafkaAdmin changes I made:

1. Stop using broker-errors for client-side problems #1639
2. Fix KafkaAdmin send to controller #1640
3. Add group coordinator lookup #1641
4. Fix describe groups #1642
5. Add list consumer groups offsets #1643
6. Set a clear default value for validate_only / include_synonyms #1645

I didn't have time yet to do tests... Given that the existing KafkaAdminClient code currently on the master branch is broken, sometimes in subtle ways, I decided fine to put them up as-is since then the code will be mostly working, and then later add tests to prevent regressions.

Frankly, the test fixtures are a bit of a mess, so I will probably instead go the route of finishing #1193, as that will both simplify the code drastically and require a few fixture changes.

And then after that tackle the tests/missing coverage.

But creating this and assigning it to myself so that I don't forget

[jeffwidman]

I am unlikely to get to this as my day job no longer involves Kafka.

PR's welcome if anyone is interested!