Adds check for missing gateway ID on gateway commands (GoogleCloudPlatform#2780)

* Adds check for missing gateway ID on gateway commands
* Adds back flaky directive

Author: gguuss

iot/api-client/mqtt_example/cloudiot_mqtt_example.py

@@ -482,6 +482,11 @@ def mqtt_device_demo(args):
 def main():
     args = parse_command_line_args()
 
+    if args.command.startswith('gateway'):
+        if (args.gateway_id is None):
+            print('Error: For gateway commands you must specify a gateway ID')
+            return
+
     if args.command == 'gateway_listen':
         listen_for_messages(
                 args.service_account_json, args.project_id,

iot/api-client/mqtt_example/cloudiot_mqtt_example_test.py

@@ -280,6 +280,7 @@ def test_gateway_listen_for_bound_device_configs(test_topic, capsys):
     assert 'Received message' in out
 
 
+@flaky
 def test_gateway_send_data_for_device(test_topic, capsys):
     gateway_id = device_id_template.format('RS256')
     device_id = device_id_template.format('noauthbind')