Allow BaseTestCase use a different conf object

The new messaging work will register options like rpc_backend and, since
the existing RPC API registers those options with CONF, we need to run
the messaging tests with a different ConfigOpts object.

It's actually probably pretty sane for all unit tests to use a per-test
ConfigOpts object rather than cfg.CONF anyway.

The kombu and zmq tests have an issue with this where they rely on being
able to call self.config() before the base class setUp() is called.
Fixing this properly is a little tricky, so for now, initialize
self.conf early to cfg.CONF just for these tests.

Change-Id: I7b2e3db7c21c511f3ab16ac866e0cc80846cbd80