Fix Fog::DNS::PowerDNS::Zones.all() and .get()

[davispuh]

I was trying

dns = Fog::DNS::PowerDNS.new({
  :host => host,
  :pdns_api_key => key
})

zones = dns.zones.all
zone = dns.zones.get('example.org')

And it was failing with

fog-powerdns-0.2.1/lib/fog/dns/powerdns/requests/get_zone.rb:37:in `get_zone': wrong number of arguments (given 1, expected 2) (ArgumentError)
fog-powerdns-0.2.1/lib/fog/dns/powerdns/models/zones.rb:21:in `get'

This is because list_zones and get_zone require a server parameter. See get_zone.rb

[davispuh]

Ping? Could someone take a look at my PRs. Thanks!