API: Don't require nameservers list for Slave zones

When creating a Slave zone, it usually doesn't make a lot of sense
to create NS records, as they'll be overwritten by the slaving mechanism
anyway.
This allows not passing 'nameservers' in at all.

Fixes PowerDNS/pdnsapi#2.
Merge pull request #1423 from zeha/api-slave-no-nameservers

API: Don't require nameservers list for Slave zones