Attach independent disks - DO NOT MERGE

This PR enables attaching of independent disks to launched VMs. It is dependent on the vcloud-core code added in https://github.com/gds-operations/vcloud-core/pull/128

In summary, it adds an :independent_disks subsection to the 'vm' config section, detailing an array of independent disks to attach.

It cannot be merged until https://github.com/gds-operations/vcloud-core/pull/128 is in Core.


Could we infer the vdc name from the vCloud Launcher configuration, e.g.:
https://github.com/mikepea/vcloud-launcher/blob/9aa6d78259a187a58b0ea3d525805541921c469e/spec/integration/launcher/data/happy_path.yaml.erb#L4

That would save some API calls.