Fix IoT parameters [(#2320)](GoogleCloudPlatform/python-docs-samples#2320)

Author: gguuss

samples/api-client/manager/manager.py

@@ -755,7 +755,6 @@ def parse_command_line_args():
             help='Path to public ES256 key file.')
     parser.add_argument(
             '--gateway_id',
-            required=True,
             help='Gateway identifier.')
     parser.add_argument(
             '--member',