Remove ConfFixture from toplevel public API

There's no need to make the fixtures, testtools, etc. libraries a
runtime requirement, so let's move it from the toplevel oslo.messaging
API so you need to explicitly import it.

Change-Id: I9e2f32a898d78489f2d8d9c218c81f35cda14e34
Merge "Remove ConfFixture from toplevel public API"