Parametrize the test description

[pbaughman]

Do something like pytests @pytest.mark.parametrize for the launch description so we can easily run tests like this one: https://github.com/ros2/system_tests/blob/a5d2c6daffe1ffd9baf39186f3b442a38fcffb35/test_communication/test/test_publisher_subscriber.py.in

Edge cases to check (WIP)

• ctrl+c during a parametrized test run should exit all runs. Note This was done by doing an early-out any time the processes under test dies before the tests finish. Maybe not optimal
• Mixing launch descriptions that require command-line arguments with ones that don't
• give a good error message if the argument in generate_test_description doesn't match the argname in the decorator
• give a good error message if parametrize has too many different args
• give a good error message if paraetrize has too few args

[pbaughman]

@hidmic I've started this on our fork, mainly so I can use the CI that's already set up. I'll do the necessary work to port it to the ros2/launch fork after I get a working prototype that you can throw darts out

[hidmic]

That's great @pbaughman, thank you.

[pbaughman]

Merged here: #39