Move node loading to separate file

So that it's easier to read.
Use vcloud-launcher configs in govuk-provisioning

vcloud-templates and vcloud-box-spinner are dead. This is a simpler version
of Nicola's yaml_input branch which:

- Uses the updated syntax of vcloud-launcher
- Only takes the required items out of each vApp entry
- Doesn't add disks - I'll create a separate story for that
- Removes support for JSON - we no longer need it

It will raise an error if you have an old `nodes.local.json` file.
Move load_nodes() out of Vagrant.configure

Having it within seems to cause it to be called twice which is evidenced by
two error messages when you have an old `nodes.local.json` file:

➜  vagrant-govuk git:(69925732-vcloud_launcher_yaml) ✗ vagrant status
ERROR: nodes.local.json is deprecated. Please convert it to YAML
ERROR: nodes.local.json is deprecated. Please convert it to YAML
Merge pull request #25 from alphagov/69925732-vcloud_launcher_yaml

[Fixes #69925732] Use vcloud-launcher configs in govuk-provisioning