Fix synchronization for vcloud-director NAT

- Previously was broken if two apps were deploying concurrently,
  because they have different PortForwarderVcloudDirector instances.
- Documents synchronisation model in NatService.
- Adds test (and tidies test config)
vcloud retry on “busy completing an operation”

- If someone else is modifying the vcloud-director Edge Gateway, 
  then our command will fail with an exception telling us
  the server was “busy completing an operation”. Therefore retry.
Merge pull request #14 from aledsage/vcloud/nat-20141219

Fix synchronization for vcloud-director NAT