Add integration test for the OIDC discovery endpoint

- Intended to be a red test in this commit; will make it go
  green in a future commit
- Enhance env.go and prepare-for-integration-tests.sh to make it
  possible to write integration tests for the supervisor app
  by setting more env vars and by exposing the service to the kind
  host on a localhost port
- Add `--clean` option to prepare-for-integration-tests.sh
  to make it easier to start fresh
- Make prepare-for-integration-tests.sh advise you to run
  `go test -v -count 1 ./test/integration` because this does
  not buffer the test output
- Make concierge_api_discovery_test.go pass by adding expectations
  for the new OIDCProviderConfig type

hack/lib/kind-config/multi-node.yaml

@@ -0,0 +1,8 @@
+kind: Cluster
+apiVersion: kind.x-k8s.io/v1alpha4
+nodes:
+  - role: control-plane
+  - role: worker
+    extraPortMappings: [{containerPort: 31234, hostPort: 12345, protocol: TCP}]
+  - role: worker
+    extraPortMappings: [{containerPort: 31234, hostPort: 12345, protocol: TCP}]

hack/lib/kind-config/single-node.yaml

@@ -0,0 +1,5 @@
+kind: Cluster
+apiVersion: kind.x-k8s.io/v1alpha4
+nodes:
+  - role: control-plane
+    extraPortMappings: [{containerPort: 31234, hostPort: 12345, protocol: TCP}]