No longer start any service by default

[solsson]

Motivation:

• This is how the Kafka distribution works.
• You'll likely want at least Zookeeper and Kafka to run from this image, so setting an entrypoint for both is more consistent.
• Once Kafka 0.11 on debian:stretch #5 is merged to master and built as solsson/kafka, environments that did the 100k pull so far might break, and I prefer them to not start, compared to for example silently reverting config.

Motivation for avoiding all config overrides:

• Support drop-in replacement of alternatives to my builds, for example cp-kafka or https://github.com/kubernetes/contrib/tree/master/statefulsets/kafka. Of course this depends on how you've configured your services, but it's a noble thought :)