Add option to not generate CDNSKEY record

We already have the ability to not generate CDS records (by configuring an empty cds-digest-types {};), but we do still always generate the CDNSKEY.

We could solve this either with:

1. Add new option cdnskey yes:no
2. Allow special value cdnskey in cds-digest-types list.

Despite it being yet another configuration option, I think the first one is preferred, rather than overloading the cds-digest-types option.