Description of problem:
upgrade_control_plane.yml removes secret-kube-etcd-client-certs volume from statefulset/prometheus-k8s.

Version-Release number of the following components:
openshift-ansible-3.11.153-2

How reproducible:
100%

Steps to Reproduce:
1. Configure etcd endpoint monitoring (https://docs.openshift.com/container-platform/3.11/install_config/prometheus_cluster_monitoring.html#configuring-etcd-monitoring_prometheus-cluster-monitoring)
2. Run upgrade_control_plane.yml playbook

Actual results:
Etcd volume entry is removed from the statefulset/prometheus-k8s manifest:

$ oc set volume statefulset prometheus-k8s | grep etcd
$

Expected results:
After upgrade playbook completion, etcd secret volume should be kept:

$ oc set volume statefulset prometheus-k8s | grep etcd
  secret/kube-etcd-client-certs as secret-kube-etcd-client-certs
    mounted at /etc/prometheus/secrets/kube-etcd-client-certs in container prometheus


Additional info: