Document URL: 
https://docs.openshift.com/container-platform/3.11/install_config/cluster_metrics.html#metrics-ansible-variables

Section Number and Name:
Specifying Metrics Ansible Variables

Describe the issue: 
The document doesn't have information about the default value when users don't specify `openshift_metrics_cassandra_nodeselector` / `openshift_metrics_hawkular_nodeselector ` / `openshift_metrics_heapster_nodeselector `.

If users don't have any specific value in their inventory, the nodeselector becomes blank value("") like this:

```
$ oc get project openshift-infra -o yaml
apiVersion: project.openshift.io/v1
kind: Project
metadata:
  annotations:
    openshift.io/node-selector: ""
...
```

It means that the pods in openshift-infra ignore defaultNodeSelector and may be deployed to any kind of node.

I think the behavior is confusing and should be documented.

Suggestions for improvement: 
The documentation about the default behavior can be added to improvement.

Additional information: