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
Worth a comment in the class' javadoc about what setup is expected (i.e. need things in brooklyn.properties). It can just point people to the javadoc of `NatServiceLiveTest.java` for details of exactly what.

Think of it as the next person to check out this project and run the tests will find that they all fail. That person needs to be given directions/pointers for the config required to get it working.