Adds support for the Security Group extension to neutron

Well, I tend to prefer having the tests complete, but that's just a personal feeling.
In an hypothetical scenario where we had to disable the create OK test, because the deserialization of the response was broken, we wouldn't have any assertion that the request is being properly generated, even when there are other tests that call the *create* method.
That's unlikely to happen, so being pragmatic the test would be ok as-is, but for completeness, I tend to prefer adding all relevant assertions to make sure everything has gone as expected.
It's perhaps just a matter of taste, and I'd be OK if the test remains as-is. Up to you!