Add an environment variable to allow forcing either the stateful or the
stateless authenticate flow.
Split the existing integration test clusters "single" and "multi" into
four new clusters: "single-stateful", "single-stateless",
"multi-stateful", and "multi-stateless", so that the integration tests
will run for both the stateful and the stateless authenticate flows.
(The "kubernetes" cluster is not currently being run, so I've left it
alone for now.)
