Simulates events so run_tests.py executes 10x faster

This restores some functionality that had been removed earlier to use
a class from Oslo. It turns out making it work with the new Oslo RPC
manager code wasn't too hard, plus the new version is hopefully easier
to follow.

Change-Id: I18e4d06359f28605cd764a10ace791997f534ed7
Implements: blueprint bring-back-event-simulator
Merge "Simulates events so run_tests.py executes 10x faster"