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
I see we still have `testOpenPortForwardingAndAdvertiseUsingNetworkName` and `testOpenPortForwardingAndAdvertiseUsingNetworkId`, but they both now look identical. Can we rename it to `testOpenPortForwardingAndAdvertise`, and delete the second one?