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
It doesn't feel quite right to treat `IllegalStateException` in a special way like this (to rethrow, where everything else is just logged). But I see why you've done it (because that is the exception thrown if the port-forwarding still exists at the end of the method). No strong feelings, other than it deserves a comment in the catch block.