Description of problem:
It seems there is no way to have the upgrade playbook picking up an already PVC or PV attached to the cassandra pod, since no matter the variable we use in the inventory the playbook keeps creating a new PVC and PV for the cassandra.

Version-Release number of the following components:
This happens from version 3.9 of OCP until the latest:

ansible --version = from 2.4.6.0 to 2.6.11
openshift-ansible = from 3.9.z to 3.11.z

How reproducible:
Every time we run the playbook to upgrade Metrics stack

Steps to Reproduce:
Variables used on the inventory:

openshift_metrics_cassandra_storage_type=pv
openshift_metrics_storage_volume_name="pvc-238ae6a6-fd52-11e8-9c5b-525400d757c5"
openshift_metrics_cassandra_pv_selector={'name': 'pvc-238ae6a6-fd52-11e8-9c5b-525400d757c5'}
openshift_metrics_cassandra_pvc_prefix=cassandra-vol2
openshift_metrics_cassandra_replicas=1

None of the combination below works, specially the 'openshift_metrics_cassandra_pvc_prefix' that should make the task 'Check to see if PVC already exists' to pick up existing PVC without relying on the prefix.

Actual results:
Running the upgrade playbook 'openshift-metrics/config.yml' creates a PVC and a dynamic PV every time.

Expected results:
When checking for any existing PVCs we should have other variables in the task to check for PVC in the project.

Additional info:
<!--snippet from the task--->
- name: Check to see if PVC already exists
  oc_obj:
    state: list
    kind: pvc
    name: "{{ openshift_metrics_cassandra_pvc_prefix }}-{{ metrics_pvc_index }}"
    namespace: "{{ openshift_metrics_project }}"
  register: _metrics_pvc
<------>

https://github.com/openshift/openshift-ansible/blob/master/roles/openshift_metrics/tasks/generate_cassandra_pvcs.yaml
https://github.com/openshift/openshift-ansible/commit/5341e36899a03bf48f0e2c2be7eb5ccb5548bda2#diff-3e36fa4bda3f7e4aad74e56576aa8aee