The current puppet-nova support for cell_v2 does not update the transport_url for a cell when it changes. In a HA deployment the rabbitmq transport url contains all of the controllers urls. As controllers are scaled up the new controllers will not be added to the this. If the original controllers are scaled down then the resulting transport url will contain no valid rabbitmq server.
*** This bug has been marked as a duplicate of bug 1443214 ***