Check KAFKA_START before JAVA_HOME

Currently, if Java hasn't yet been set up when this package is installed,
installation will fail when the init script checks for JAVA_HOME.

Instead, check for KAFKA_START first---it will be 'no' immediately upon
installation---and abort cleanly before failing to find Java.
Merge pull request #1 from patricklucas/check_kafka_start_before_java

Check KAFKA_START before JAVA_HOME