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 think this should probably return `Predicate<NatRuleType>` instead.

(Note it is the job of the method that takes a predicate as a parameter to declare that it takes a `Predicate<? super NatRuleType>`)