Bring back the event simulator for Tox tests.

In the past, the tox tests ran using "simulated events," meaning eventlet threads weren't used. However, during a recent bit of refactoring to use an Oslo RPC base class, this feature was removed as it was incompatible with other changes.

I've done some research and it turns out this was unnecessary. It is possible for the event simulator to exist alongside everything else. Additionally, the code to set up simulated events, which was previously somewhat hard to follow, has been reduced to a single module plus two lines in run_tests.py (this includes the import).

Some may ask, why even run the event simulator? The simple answer is because it speeds up the test runs by a factor of ten.

Without event simulator: 75.394s
   With event simulator: 7.573s

Because we run the tests with XML as well, this could dramatically reduce the time it takes to get tox results back from Jenkins.