DIGITALOCEAN: Enable get-zones "all" feature (implement ListZones())

[RickyGrassmuck]

Added a ListZones() function to the Digitalocean provider to satisfy the ZoneLister interface.

Previous behavior

> dnscontrol get-zones do_provider - all
provider type do_provider:- cannot list zones to use the 'all' feature

After change

> /workspaces/Misc/dnscontrol/dnscontrol get-zones do_provider - all
$ORIGIN example.com
$TTL 300
@          1800  IN NS    ns1.digitalocean.com.
           1800  IN NS    ns2.digitalocean.com.
           1800  IN NS    ns3.digitalocean.com.
  [...truncated...]