Don't keep a single global TRANSPORT object

To keep the global state of messaging like the old olso-incubator
library does, we have created a global TRANSPORT object.

But since the test use the fake:// driver of oslo.messaging in tests,
this transport is shared between tests, but some tests use 'fake://',
other the default one, and someother disable the transport.

This change ensures that a different transport is used for
code tests.

Change-Id: I22317527cc4fb44ea1fb9642586e8cdcbc97030b