Use spin_until_future_complete instead of spin_some in parameters_event demo

[ivanpauno]

From ros2/rclcpp#844, spin_some collects pending executables just once, and executes all of them.

That broke this example, because the following happens.
The clients requests aren't handled by the services until spin is called.
The parameter event is published by the service when handling the requests.

If executables are not collected more than once, on_parameter_event callbacks won't be ever called (the only ones called are the ones from the previous declare_parameter calls).

The bad thing, is that now the user have to terminate the example by using ctrl-c (maybe, add a log saying that).

Fixes: https://ci.ros2.org/view/nightly/job/nightly_linux_release/1428/testReport/junit/demo_nodes_cpp/TestExecutablesTutorial/test_processes_output/.

[wjwwood]

I don't think we should change the behavior of this demo (now it will block until ctrl-c where as before it would close on its own).

Instead, we should setup a future, complete it based on some condition, and use spin_until_future_complete.

[wjwwood]

Or you could spin in a thread, and call shutdown after some condition is met, if you want to avoid a future.

[ivanpauno]

I don't think we should change the behavior of this demo (now it will block until ctrl-c where as before it would close on its own).

Instead, we should setup a future, complete it based on some condition, and use > spin_until_future_complete.

I also thought about that, but it makes the example more complex.
The only condition I can think is: the callback was called 10 times.

Changes requires: ros2/rclcpp#971

[ivanpauno]

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