As the first step towards removing rpc_backend, we need all of the test
fixtures to stop setting/using it via ConfFixture.transport_driver. Add
ConfFixture.transport_url, and move the tests over to setting it rather
than transport_driver.
Partial-Bug: #1712399
Change-Id: I6e2ae2b511fdc52790d5755a9aecaf2f427ab706