[RFE] make zone_update_keys referencable

[OlafLostViking]

Currently, I add a zone_update_key for each zone I need to update. Since I'm currently hosting multiple domains on the same name server using the same TSIG key, I just replicate the entry multiple times. Not that much of a problem, but offering a way to reference a labeled zone_update_key instead of implicitley always use one with the domain name could further tighten the config file.