#293 UpdateLB stub and test case

I don't like it. We will host, say, 100k domains. The config object will be huge. Instead, I think, we should update nginx configuration incrementally. Something like that:

```java
nginx.update("www.example.com", 80, "ec2-67-87-90-33.amazonaws.com", 34567);
```

This will mean that we should start forwarding `www.example.com:80` to `ec2-67-87-90-33.amazonaws.com:34567`.

We will have a problem later with domain names that disappear, but we continue to forward them. And we will have to introduce something like `nginx.remove(...)` method. But this is for the future.

So, my point is that the `Nginx` object should be write-only. Nobody should be able to read the configuration. It's just not necessary.