[topology] Add optional default for topologyConstraint LabelSelector

[olliewalsh]

Replicate the defaulting behaviour used in cluster-level default constraints https://kubernetes.io/docs/concepts/scheduling-eviction/topology-spread-constraints/#cluster-level-default-constraints

[fmount]

@olliewalsh tested and works as expected (no regressions in the previous code that we tested). If you can take a look at openstack-k8s-operators/glance-operator#670 (comment) I was wondering if similarly to what we did for DistributePods [1], we can make the following:

// GetGlanceAPILabelSelector - Returns a metav1.LabelSelector for a given GlanceAPI
func GetGlanceAPILabelSelector(instance *glancev1.GlanceAPI) metav1.LabelSelector {
	return metav1.LabelSelector{
		MatchExpressions: []metav1.LabelSelectorRequirement{
			{
				Key:      glance.GlanceAPIName,
				Operator: metav1.LabelSelectorOpIn,
				Values: []string{
					fmt.Sprintf("%s-%s-%s", glance.ServiceName, instance.APIName(), instance.Spec.APIType),
				},
			},
		},
	}
}

a generic one, that can be called from this module.
This way we can avoid duplicating it across the operators, and we can provide

common.AppSelector, []string{<service>.ServiceName`}

as input for that function.
You could then call it with something like topology.GetDefaultTopologySelector(common.AppSelector, []string{<service>.ServiceName}) and pass it down to topology.EnsureTopologyRef(.

Do you want to push an update for this before we land it?

[1] https://github.com/openstack-k8s-operators/glance-operator/blob/main/pkg/glanceapi/statefulset.go#L296
[2] olliewalsh/glance-operator@8386bff

[fmount]

/lgtm