If you want to create an `OpenStackNetConfig` CR with a blank `domainName`, creating the CR will fail with the error message: 
admission webhook "vopenstacknetconfig.kb.io" denied the request: domainName must include a top-level domain and at least one subdomain
It appears that empty/blank `domainName`s are allowed, given the CRD field defition: https://github.com/openstack-k8s-operators/osp-director-operator/blob/91f01844715efb0543ca07c35a1559baa71167ef/api/v1beta1/openstacknetconfig_types.go#L118-L121.  But if left blank, the value is defaulted to `localdomain`, which then fails the validation check that comes next: https://github.com/openstack-k8s-operators/osp-director-operator/blob/91f01844715efb0543ca07c35a1559baa71167ef/api/v1beta1/openstacknetconfig_webhook.go#L421-L423.