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
applied Houndci comments
applied houndci comment on line length
changed default value for ip param in delete_dnat
Merge pull request #58 from dice-cyfronet/wrg_fix

added ip param in delete_dnat method signature