[#65619490] NatService generator dependency injection - redux

Re-work of https://github.com/alphagov/vcloud-edge_gateway/pull/11 to be easier to understand.

* Change NatService generator to no longer call out to the EdgeGateway API.
* Use the new Core::EdgeGateway.interfaces call to supply a list of EdgeGateway network details to the generators
* Re-order the NatService generator arguments to make more sense.