tests/kafka_cli_consumer: using graceful shutdown when stopping consumer

When consumer is being stopped it may send `LeaveGroup` request to the
broker. Using clean shutdown (SIGTERM) to give the consumer process
oportunity to finish gracefully.

Signed-off-by: Michal Maslanka <michal@vectorized.io>

tests/rptest/services/kafka_cli_consumer.py

@@ -93,4 +93,4 @@ def wait_for_messages(self, messages, timeout=30):
 
     def stop_node(self, node):
         self._stopping.set()
-        node.account.kill_process("java", clean_shutdown=False)
+        node.account.kill_process("java", clean_shutdown=True)