Apply node_selector for pod_spec_mutator

[Aachunella]

/kind feature

Describe the solution you'd like
When I use the fairing we couldn't apply node selector for pod_spec_mutator easily.
I could designate node selector by a function,

def set_node_selector(node_selector):
    def _set_node_selector(kube_master, pod_spec, namespace):
        if node_selector is not None:
            pod_spec.node_selector = node_selector
        
    return _set_node_selector

Anything else you would like to add:
It would be better to add a function to kubernetes/utils.py.

Can I make a PR for this function?