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
    it { expect('www.google.com').to have_dns.with_type('A').config(nameserver: s) }
  end
end
```

TODOs:
- [x] add explanation about config chain README.md
- [x] add test cases
- [x] add impl.
- [x] pass test cases or some case
I completely agree with you.
It is unnecessary. I deleted it.