Handle the publish to zone file case with a signed review hook.

[ximon18]

OpenDNSSEC can be configured to write signed zones to disk. Cascade doesn't support this.

Currently ods2cascade outputs the following advice in such a case:

Signed zones will NOT be written to disk. Cascade only supports publication of signed zones via XFR. You will need a secondary nameserver or some other tool receive/fetch new signed zone versions via XFR.

We could modify ods2cascade to generate a configuration that uses a signed review hook to dig AXFR > /path/to/save/zone the signed zone to disk (and generate the hook script too?).