Framework for testing Magnum scalability

Magnum is currently untested in large scale deployment.
This is an issue that may inhibit adoption by Cloud providers and large scale production users.

From the discussion at the design session in Tokyo, a framework will be set up to track Magnum scalability using the OpenStack Innovation Center for the test platform and Rally to drive the tests.

At the high level, the following steps are needed:

1. Define the scenarios to track

2. Develop the Rally plugin and run configuration (separate blueprint linked here)

3. Run the tests at regular intervals and save the Rally reports in Magnum repo

4. Document the full process and summary of result

5. Debug failure due to scalability and fix bottleneck

Since we will be submitting patches in the Rally project, a blueprint has been opened in Rally:
https://blueprints.launchpad.net/rally/+spec/benchmark-scenarios-for-magnum