Add a unit testing configuration fixture

The configuration options registered by oslo.messaging should not be
directly relied upon by users of the library, since those config option
names could change in future.

Add an API which allows API users to override specific configuration
options e.g.

    self.messaging_conf = self.useFixture(messaging.ConfFixture(cfg.CONF))
    self.messaging_conf.transport_driver = 'fake'

Change-Id: If0d837e1b86e3b04237fde522551cfb81505a543
Merge "Add a unit testing configuration fixture"