On the road to Scala 2.12, we'll need to make Kafka 0.8 support optional in the build, because it is not available for Scala 2.12.
https://github.com/apache/spark/pull/19134 adds that profile. I mention it because this means that Kafka 0.8 becomes "opt-in" and has to be explicitly enabled, and that may have implications for downstream builds. Yes, we can add <activeByDefault>true</activeByDefault>. It however only has effect when no other profiles are set, which makes it more deceptive than useful IMHO. (We don't use it otherwise.) Reviewers may want to check my work especially as regards the Python test support and SBT build. Another related question is: when is 0.8 support deprecated, removed? It seems sudden to remove it in 2.3.0. Maybe deprecation is in order. The driver is that Kafka 0.11 and 1.0 will possibly require yet another variant of streaming support (not sure yet), and 3 versions is too many. Deprecating now opens more options sooner.
