Gateways: make the --device_id parameter required only for commands that need it

[stevebriskin]

https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/iot/api-client/beta-features/gateway/gateway.py#L597 requires the --device_id for all invocations but some calls ( list_devices_for_gateway and list_gateways) do not need it.

[gguuss]

We could probably benefit from refactoring the commands all at once as there are more than just the device ID that should be optional.

[gguuss]

Should be resolved once we've merged this PR.