Add configuration for running flake8 tests.

[nuclearsandwich]

ros-infrastructure/rosdep#930 changes how flake8 is invoked for rosdep and @cottsay and I both prefer this pattern (largely because it prevents us from supporting a broad base of flake8 versions since their API is not intentionally public/stable).

Since we don't currently have flake8 running in tests here, let's use it as a template to add them here.