Improve api for trigger step

Trigger step was still difficult to subclass in order to add dynamic scheduler and property choosing.
Now it is much easier

While documenting this new API, it also improves a lot the doc for this step.

With this, you don't need to copy paste the entire step in order to implement travis.yml controlled parallel builds:
https://github.com/tardyp/buildbot_travis/blob/master/buildbot_travis/steps/spawner.py

This method returns ..