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 wonder about just a `LOG.warn; return;` here. If it doesn't have a valid `MachineLocation`, that might be because it didn't start up fully. But maybe it's up to the caller to catch the exception and continue. So fine with leaving as is.