Add HNC postsubmits and periodics

Run HNC's end-to-end tests in a Kind cluster, using a custom-built image
defined in
https://github.com/kubernetes-sigs/multi-tenancy/tree/master/incubator/hnc/hack/prow-e2e.

Note that I've set the periodics to run every 12h; this is temporary
until we've seen a few passing runs. After that point, I'll reduce the
frequency to 48h (i.e. every two days).

Tested: on my local workstation, the custom-built image successfully
runs the latest e2e tests on Kind following the instructions in the HNC
repo (see README at the link above). I have no way to test that the Prow
configs are correct as we've cobbled them together from other tests'
examples.

config/jobs/kubernetes-sigs/wg-multi-tenancy/hnc-e2e.yaml

@@ -0,0 +1,60 @@
+postsubmits:
+  kubernetes-sigs/multi-tenancy:
+  - name: hnc-postsubmit-test
+    annotations:
+      testgrid-dashboards: wg-multi-tenancy-hnc
+      testgrid-tab-name: postsubmit-tests
+    decorate: true
+    always_run: false
+    decoration_config:
+      timeout: 1h
+    max_concurrency: 5 # No idea what this should be for HNC
+    path_alias: sigs.k8s.io/multi-tenancy
+    run_if_changed: "incubator/hnc/.*"
+    labels:
+      preset-kind-volume-mounts: "true"
+      preset-dind-enabled: "true"
+    spec:
+      containers:
+      - image: gcr.io/k8s-staging-multitenancy/hnc-postsubmit-tests:latest
+        # The default command in the image runs the tests
+        securityContext:
+          privileged: true # Required for docker-in-docker
+        resources:
+          requests:
+            cpu: 1
+            memory: "4Gi"
+
+# The periodics are the same as the postsubmits, but we run them occasionally
+# to ensure there's no flakiness during periods where there aren't lots of
+# submissions. Please keep these two exactly in sync, except for the first
+# couple of lines (e.g. name, annotations, interval).
+periodics:
+  kubernetes-sigs/multi-tenancy:
+  - name: hnc-periodic-test
+    annotations:
+      testgrid-dashboards: wg-multi-tenancy-hnc
+      testgrid-tab-name: periodic-e2e-tests
+      testgrid-alert-email: aludwin@google.com # will change to a group when it's stable
+      testgrid-num-failures-to-alert: "1"
+    interval: 12h # Will reduce to 48h as soon as we know it's working (@adrianludwin, Feb 2021)
+    decorate: true
+    always_run: false
+    decoration_config:
+      timeout: 1h
+    max_concurrency: 5 # No idea what this should be for HNC
+    path_alias: sigs.k8s.io/multi-tenancy
+    run_if_changed: "incubator/hnc/.*"
+    labels:
+      preset-kind-volume-mounts: "true"
+      preset-dind-enabled: "true"
+    spec:
+      containers:
+      - image: gcr.io/k8s-staging-multitenancy/hnc-postsubmit-tests:latest
+        # The default command in the image runs the tests
+        securityContext:
+          privileged: true # Required for docker-in-docker
+        resources:
+          requests:
+            cpu: 1
+            memory: "4Gi"