Refactor zones

- Rename "zonefile_setting" to "zone"
- Remove version

#475

spec/models/dns/zone_spec.rb

@@ -0,0 +1,13 @@
+require 'rails_helper'
+
+RSpec.describe DNS::Zone do
+  describe '::origins' do
+    before :example do
+      expect(described_class).to receive(:pluck).with(:origin).and_return('origins')
+    end
+
+    it 'returns origins' do
+      expect(described_class.origins).to eq('origins')
+    end
+  end
+end