Add a new PowerDNS backend based on MiniDNS

This is stage one of phasing out the old PowerDNS driver, as we move
through the Kilo cycle and mDNS/pools become reality, the original driver
can be removed.

Implements-Blueprint: transition-powerdns-to-mdns
Change-Id: I7a3aa0a894e2c0a4fa565e8d58cccba3644e0cac