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 for including in the log message a `(throwing)` at the end - i.e. `LOG.error(builder.toString() + " (rethrowing)");`.

Note that we'll have an extra comma at the end, but don't worry about that.