Refactor Tests to use Mixins

We've made some progress, but the testing in Keystone could still be cleaned up.

The changes we did (making ports dynamic, removing dependency on sample data, and separating out test types) resulted in mixing a bunch of http-ish abstractions with a bunch of python-ish abstractions (which depend on the http-ish stuff), and that needs to be fixed.

There's also a bunch of methods that follow restful terminology, e.g. post_user, which understand keystone urls, etc... and then there's a bunch of pythonish methods that use those, e.g. create_user, that build restful objects for you.

Thoughts:
class ClientTestCase(HttpTestCase, ApiTestCase)
class FunctionalTestCase(WsgiTestCase, ApiTestCase)
where HttpTestCase and WsgiTestCase implement whatever they need to make requests happen separately.

Fixture loading also needs improving. Maybe:
- class test.common.BootstrapMixin() -> calls new keystone-manage commands?

Not *every* test would need that mixin (client tests checking version response, for example)