Set `PortsPerNetworkInterface` when creating a `NATGateway`

[ushabelgur]

Proposed Changes

• provide the configuration of the portsPerNetworkInterfaces of a NATGateway by introducing a new NATConfig field to the Infrastructure configuration.
• NATConfig will allow configuration of the portsPerNetworkInterfaces of a NATGateway by setting PortsPerNetworkInterface(Same field will be used as MinPortsPerNetworkInterface )
• NATConfig will allow to automatically determine a meaningful value if MaxPortsPerNetworkInterface value is also set.

Fixes #545