[WIP] Add config chain for DNS server related configuration

I test primary/secondary DNS servers if both servers work.
Now, we need to prepare `config.yml` for each server.

I want to use config chain for each server without `config.yml`.
Like this spec.

```ruby
%w(8.8.8.8 8.8.4.4).each do |s|
  describe s do
    describe 'www.google.com' do
      it { is expected.to have_dns.with_type('A').config(nameserver: s) }
    end
  end
end
```

TODOs:
- [ ] add explanation about config chain README.md
- [ ] add test cases
- [ ] add impl.
- [ ] pass test cases or some case