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
do you think something like 
```
// meant to catch ISE coming from NatService.checkPublicIp
            if (e.toString().contains("is not valid. Public IP must fall in the following ranges:")) {
                throw Throwables.propagate(e);
            } else {
                // this shouldn't happen, so log-and-return
                LOG.error("Failed creating port forwarding rule on "+this+" to "+target, e);
                return HostAndPort.fromParts(publicIp, publicPort);
            }
```
would help more?