Switch to using ArgoCD to deploy cluster manifests instead of kubectl

[upodroid]

When changes to the cluster configuration is made, a prow postsubmit job runs kubectl apply -f prow/cluster/build to deploy the config, but this problematic for a few reasons:

• No pruning support. I deleted a Service in Deploy new monitoring stack #3360 but it still around and requires manual cleanup by a human
• It is kind of slow: https://prow.knative.dev/view/gs/knative-prow/logs/post-test-infra-deploy-prow/1532834481885417472
• Resource apply order complexity.
• Using helm charts is messy. Someone needs to do $ helm template --include-crds --output-dir ./output_dir external-secrets/kubernetes-external-secrets instead of

# Random Example
apiVersion: argoproj.io/v1alpha1
kind: Application
metadata:
  name: certmanager-prod
  namespace: argocd
spec:
  destination:
    namespace: cert-manager
    name: prod
  project: default
  source:
    chart: cert-manager
    helm:
      parameters:
      - name: installCRDs
        value: "true"
      releaseName: cert-manager
    repoURL: https://charts.jetstack.io
    targetRevision: v1.6.1
  syncPolicy:
    automated:
      prune: true
      selfHeal: true
    syncOptions:
    - CreateNamespace=true

• Updating third party tools loaded via Helm will be simpler in addition to leaner config.

/cc @kvmware

If we are happy with this, let me know and i'll bootstrap argocd in the prow cluster.

[dprotaso]

just an FYI

1. kubectl has prune support
2. In serving we use kapp cli which is like a smarter kubectl - that solves ordering and waiting without the extra infrastructure

that's all I have - haven't used Argo myself so I'm neutral