Based on our experiences today with GKE, it will be easier for our users
to configure Ingress health checks if the healthz endpoint is available
on the same public port as the OIDC endpoints.
Also add an integration test for the healthz endpoint now that it is
public.
Also add the optional `containers[].ports.containerPort` to the
supervisor Deployment because the GKE docs say that GKE will look
at that field while inferring how to invoke the health endpoint. See
https://cloud.google.com/kubernetes-engine/docs/concepts/ingress#def_inf_hc
