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
Personal preference: I'd have gone for `for (IpRangeType ipRangeType : subnetParticipation.getIpRanges().getIpRange()) { ...`, and use a `break;` if found (with the existing `found = ...` code as well obviously. But no strong feelings. Whatever you think is easiest to read.