Automatic merge from submit-queue (batch tested with PRs 67764, 68034, 67836). If you want to cherry-pick this change to another branch, please follow the instructions here: https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md.

apiserver: make not-found external-apiserver-authn configmap non-fatal

As client-ca and requestheader-client-ca is optional in the external-apiserver-authentication config file and components like kube-controller-manager and kube-scheduler won't need that anyway, we better make it non-fatal if the configmap is not found in the cluster.

Consumer counter-part PR to https://github.com/kubernetes/kubernetes/pull/67694.

```release-note
Don't let aggregated apiservers fail to launch if the external-apiserver-authentication configmap is not found in the cluster.
```