Attach independent disks (redo of #75)


This PR enables attaching of independent disks to launched VMs. It is dependent on the vcloud-core code added in gds-operations/vcloud-core#128

In summary, it adds an :independent_disks subsection to the 'vm' config section, detailing an array of independent disks to attach.

NB: This is a re-submission of #75 with a temporary IndependentDisk fixture for the attach integration test.