This doesn't fully implement SSL fixtures, but as a first step it should help with automatically generating required certificates / keystores / etc. My hope is that this helps generate more community support for SSL testing!

The current client attempts to bootstrap once during initialization, but if it fails there is no second attempt and the client will be inoperable. This can happen, for example, if an entire cluster is down at the time a long-running client starts execution.

This commit attempts to fix this by removing the synchronous bootstrapping from `KafkaClient` init, and instead merges bootstrap metadata with the cluster metadata. The Java client uses a similar approach. This allows us to continue falling back to bootstrap data when necessary throughout the life of a long-running consumer or producer.

Fix #1670

Small change to avoid doing dns resolution when running local connection tests. This fixture always returns a broker on localhost:9092, so DNS lookups don't make sense here.