I'm not 100% certain that this change is kosher. I'm not sure what the rationale was, but CydnsRecord assumes that its subclasses have label and domain fields regardless of whether they inherit from LabelDomainMixin or not. Moving this call from CydnsRecord to LabelDomainMixin might cause classes that inherit from CydnsRecord but not LabelDomainMixin to not set their FQDN unexpectedly. Incidentally, there is another `set_fqdn` call in `CydnsRecord.__str__()` that was not moved, so this change also seems inconsistent to me.