Support matchExpressions for selector

[tvandinther]

What problem are you facing?

I want to select based on the existence of a label regardless of its value.

How could this Function help solve your problem?

Offer a the standard matchExpressions selector API common to Kubernetes.

matchExpressions:
    - { key: tier, operator: In, values: [cache] }
    - { key: environment, operator: NotIn, values: [dev] }
    -  { key: environment, operator: Exists }