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.


I had considered this originally, but due to the way we pass config subsections down thru VappOrchestrator -> VmOrchestrator -> IndependentDiskOrchestrator this is not possible without passing the vdc_name (or vapp_config) down to VmOrchestrator.

TBH though, I think this is cleaner -- it means IndependentDiskOrchestrator can function independently of these calling classes. There's no need to pass vdc_name in, since the we can look it up from the required vm object we pass in.

More API calls for sure, but they are pretty cheap lookups, and we're guaranteed of getting the right information.