Make sure Kafka is dead after the test

Sometimes the next test fails to start as Kafka is still listening on
port 4008. To make sure it isn't, forcibly kill the java process at the
end of the test.

system-test/maxtest/include/maxtest/kafka.hh

@@ -83,6 +83,7 @@ private:
             false,
             "kafka/bin/kafka-server-stop.sh;"
             "kafka/bin/zookeeper-server-stop.sh;"
+            "pkill -9 java;"
             "rm -rf /tmp/zookeeper /tmp/kafka-logs;");
     }