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.
Depend on any java7 jre/jdk
Use log4j-1.2 instead of log4j-1.2.16

/usr/share/java/log4j-1.2.jar is a symlink to the latest installed
log4j 1.2. This fixes an issue where we have log4j-1.2.17 but not 1.2.16.