Use vcloud-launcher configs from Preview

Instead of Production because:

- they contain "upcoming" machines like `transition-postgresql-master-1`
  which haven't been created in Production yet.
- have a more reasonable count of machines whilst still being
  representative, like two instead of three `backend-N` machines.
Merge pull request #29 from alphagov/use_preview_vcloud_launcher_defs

Use vcloud-launcher configs from Preview