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
We don't want to `acquirePublicPort` here. That will give us a brand new public port, so `getService().closePortForwarding` won't find it. Maybe `optionalPublicPort` should not be optional. And maybe there should be an overloaded method that takes a `EntityAndAttribute<HostAndPort> mappedPort`, but not sure about that.