added ip param in delete_dnat method signature

https://github.com/dice-cyfronet/atmosphere/issues/57

VM has an ip and DNAT redirections were created for this ip.
When ip is updated it is required to destroy redirections.
Atmo used to try to destroy DNAT using updated ip
instead of the old ip. This was fixed in this commit.

- introduced ip param to delete_dnat method signature
- updated call to this method
- created spec test
Maybe following solution is a little bit cleaner:

```ruby
def delete_dnat(ip = self.ip)
  compute_site.dnat_client.remove(ip)
end
```