Attach independent disks

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.