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'd probably delete this comment - my rule of thumb for commenting (and reading comments) is that a comment will most likely apply to the subsequent block of code. This comment just applies to the one subsequent line (and just duplicates the method name), with the next line being entirely different.

I'd leave `checkPublicIp` with the previous code block, and then have a line gap followed by a comment like `// generate the new rule`.