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
vcloud NAT rules: incorporate review comments/tidy
Merge pull request #16 from aledsage/upstream-copy/fix/remove-nat-rules

Add/remove nat rules in vcloud