Removing ability to change default cassandra_pvc_prefix based on metrics volume name

roles/openshift_metrics/defaults/main.yaml

@@ -54,7 +54,7 @@ openshift_metrics_master_url: https://kubernetes.default.svc
 openshift_metrics_node_id: nodename
 openshift_metrics_project: openshift-infra
 
-openshift_metrics_cassandra_pvc_prefix: "{{ openshift_metrics_storage_volume_name | default('metrics-cassandra') }}"
+openshift_metrics_cassandra_pvc_prefix: metrics-cassandra
 openshift_metrics_cassandra_pvc_access: "{{ openshift_metrics_storage_access_modes | default(['ReadWriteOnce']) }}"
 
 openshift_metrics_hawkular_user_write_access: False