Switch to oslo_messaging.ConfFixture.transport_url
Switch to oslo_messaging.ConfFixture.transport_url

oslo_messaging's rpc_backend setting, which is set by
ConfFixture.transport_driver has been deprecated since Newton. To allow
oslo_messaging to remove it, switch to setting transport_url instead.

Change-Id: Ie37b20d1be6c177f2f1c26cc473b632d7d934c05
Partial-Bug: #1712399

stackforge/watcher,openstack/watcher,openstack/watcher,stackforge/watcher