Support for PodSecurityPolicy

[yu2003w]

If PodSecurityPolicy is enabled in k8s cluster, deployment autoscaler and activator will not be available without proper PSP.

Currently we could inject PSP when creating CR as below,

apiVersion: v1
kind: Namespace
metadata:
 name: knative-serving
---
apiVersion: extensions/v1beta1
kind: PodSecurityPolicy
metadata:
  name: knative-serving-psp
  labels:
    serving.knative.dev/release: devel
    serving.knative.dev/controller: "true"
spec:
  allowPrivilegeEscalation: true
  allowedCapabilities:
  - 'NET_ADMIN'
  fsGroup:
    rule: RunAsAny
  privileged: true
  runAsUser:
    rule: RunAsAny
  seLinux:
    rule: RunAsAny
  supplementalGroups:
    rule: RunAsAny
  volumes:
    - '*'
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: knative-serving-psp
  labels:
    serving.knative.dev/controller: "true"
rules:
- apiGroups:
  - extensions
  resourceNames:
  - knative-serving-psp
  resources:
  - podsecuritypolicies
  verbs:
  - use
---
apiVersion: v1
kind: Secret
metadata:
  name: regsec
  namespace: knative-serving
data:
  .dockerconfigjson: eyJhdXRocyI6IHsiaWNwZGV2LmljcDo4NTAwIjogeyJhdXRoIjogIllXUnRhVzQ2WVdSdGFXND0ifX19Cg==
type: kubernetes.io/dockerconfigjson
---
apiVersion: serving.knative.dev/v1alpha1
kind: KnativeServing
metadata:
  name: knative-serving
  namespace: knative-serving
spec:
  registry:
    override:
    imagePullSecrets:

Could serving-operator support PodSecurityPolicy?
If so, user only need to enable PSP support when they create CR and PSP related stuff could be created by serving-operator automatically.

Proposal:

apiVersion: serving.knative.dev/v1alpha1
kind: KnativeServing
metadata:
  name: knative-serving
  namespace: knative-serving
spec:
  registry:
    override:
    imagePullSecrets:
  podSecurityPolicySupport: false