policy: Add policy to prevent host network usage

The `policy` Kustomize project defines various cluster-wide security
policies.  Initially, this includes a Validating Admission Policy that
prevents pods from using the host's network namespace.

policy/kustomization.yaml

@@ -0,0 +1,5 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+- disallow-hostnetwork.yaml