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/disallow-hostnetwork.yaml

@@ -0,0 +1,43 @@
+apiVersion: admissionregistration.k8s.io/v1beta1
+kind: ValidatingAdmissionPolicy
+metadata:
+  name: disallow-hostnetwork
+spec:
+  matchConstraints:
+    resourceRules:
+    - apiGroups:
+      - ''
+      apiVersions:
+      - v1
+      operations:
+      - CREATE
+      - UPDATE
+      resources:
+      - pods
+  validations:
+  - expression: >-
+      !has(object.spec.hostNetwork) || !object.spec.hostNetwork
+    message: >-
+      Pods must not use hostNetwork: true
+
+---
+apiVersion: admissionregistration.k8s.io/v1beta1
+kind: ValidatingAdmissionPolicyBinding
+metadata:
+  name: disallow-hostnetwork-binding
+spec:
+  policyName: disallow-hostnetwork
+  validationActions:
+  - Deny
+  matchResources:
+    namespaceSelector:
+      matchExpressions:
+      - key: kubernetes.io/metadata.name
+        operator: NotIn
+        values:
+        - calico-system
+        - democratic-csi
+        - keepalived
+        - kube-system
+        - music-assistant
+        - tigera-operator