Some of the heat unit tests import code from a private part of
oslo.messaging. With the namespace package change, that private part was
no longer being exposed. This change introduces the needed module so we
can release oslo.messaging with the namespace changes without breaking
heat, to give the heat team time to update their tests.
Partial-Bug: #1410196
Change-Id: Ib3ea106646f75b17665877b08ccbd382c5698714