Random NAT IP assignment in `NetworkInterface`

[afritzler]

Describe the bug
Currently we are having a bug that if more than one NATGateway is present in the landscape, the NetworkInterface is referencing a wrong gateway. This can be observed by a wrong NAT IP assignment in the NetworkInterface spec.

To Reproduce
Create more than one NATGateway, create a NetworkInterface and observe which NATGateway is associated. Repeat and observe a random behavior of gateway assignments.

[afritzler]

Fixed with #241