add remove NAT rules

add support for closePortForwarding for PortForwarderVcloudDirector
refactor VcloudDirectorSubnetTierLiveTest to create a VM for the test
modify NatService: networkId is no more required by the user
Don't want to check args or log here - it was done in the calling method `closePortForwarding`. Doing it here would mean we'd see the log message for every attempt.