Store kafka data inside /opt/kafka/data regardless of image config

[solsson]

Thanks to @allenj in #11 (comment)

First commit fixes this regardless of the kafka image used.

[solsson]

With recent builds of the suggested image solsson/kafka-persistent:0.10.1 you get this fix through the default server.properties, so the override isn't necessary. Commits:

• 3c11c9f
• c55b91b

[solsson]

With the Kafka 0.11 branch #30 we're back to a generic image, and thus we need this PR. I've rebased and merged in a83683f.