rcl should use test_msgs instead of example_interfaces

[mikaelarguedas]

As example_interfaces are used only for examples and demo and their removal / modification should not impact the test or release of rcl

[mjcarroll]

In the cases where a service is used (AddTwoInts), did you want to replace it with an empty service or add a new service to the test_msgs package?

[mikaelarguedas]

I was thinking of merging ros2/rcl_interfaces#25 that adds a pretty comprehensive set of services