Pod can override Namespace default nodeSelector

1. Proposed title of this feature request
Pod can override Namespace default nodeSelector

2. What is the nature and description of the request?
OpenShift provides the ability to configure a 'default' nodeSelector value for an entire Namespace using the `openshift.io/node-selector` annotation.

When a Pod is created with an additional/different NodeSelector, the `.pod.spec.nodeSelector` and `openshift.io/node-selector` values are merged into an array. This means it is impossible to create Pods in the Namespace which are not bound to nodes containing `openshift.io/node-selector` value.

There should be a way to override the `openshift.io/node-selector` on a per-Pod basis so a default value can be set but Pos which do not conform to this requirement can still be placed in the same Namespace.

3. Why does the customer need this? (List the business requirements here)
Easier organising of the OpenShift Platform workloads

4. List any affected packages or components.
OpenShift Kube APIServer

Additional Notes:

The merge for the annotation and `.pod.sppec.nodeSelector` happens hereP:
https://github.com/openshift/kubernetes/blob/5497c42cfe31180531f3fd12da260bf97bf3486f/openshift-kube-apiserver/admission/scheduler/nodeenv/admission.go/#L75-L101