tests/kafka_cli_consumer: added wait_for_started method

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

tests/rptest/services/kafka_cli_consumer.py

@@ -91,6 +91,15 @@ def wait_for_messages(self, messages, timeout=30):
                    timeout,
                    backoff_sec=2)
 
+    def wait_for_started(self, timeout=10):
+        def all_started():
+            return all([
+                len(node.account.java_pids("ConsoleConsumer")) == 1
+                for node in self.nodes
+            ])
+
+        wait_until(all_started, timeout, backoff_sec=1)
+
     def stop_node(self, node):
         self._stopping.set()
         node.account.kill_process("java", clean_shutdown=True)