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.

Change-Id: I6b38ac4e499647cb603356ee25829210a2624530
Check KAFKA_START before JAVA_HOME (kafka-mirror)

This applies the same change from bab68f62 to the kafka-mirror init
script.

Change-Id: I2165b372acfc49c6ee220ff9b43ad2b4876e04ed