Allow using PowerDNS-subset API without cryptokeys endpoint

[dancojocaru2000]

Describe the bug
When trying to use Servfail (which implements a subset of the PowerDNS API), dnscontrol preview fails because the cryptokeys endpoint is not available

To Reproduce
Steps to reproduce the behavior:

1. Create creds.json with POWERDNS type
2. Create dnsconfig.js with domain on Servfail
3. Run dnscontrol preview
4. Error getting corrections (servfail): not found: https://beta.servfail.network/api/v1/servers/ns1.famfo.xyz./zones/dcdev.ro/cryptokeys

Expected behavior
dnscontrol preview should work.

DNS Provider

• POWERDNS

[cafferata]

Ping @jpbede, the maintainer of the PowerDNS provider