Feature/kvm external disk

Implement external-disk parameter being an ordered list of parameters type and path.

type is one of qcow2 and ceph.

If path starts with / is considered absolute, otherwise relative to the instance root.

path is never automatically created, if it does not exist or it is not available to the user (read/write), it shall result with failing start of the VM and most possibly missing promise.

Hide external-disk-number, external-disk-size and external-disk-format, but keep it working to ease eventual manual migration by the user.

Tasks:

• (this is totally separated) fully testing existing features of external-disk-number, external-disk-size and external-disk-format with enabled external_storage_list in slapos.cfg
• make new implementation with test
• hide old implementation from schemas (keep it working)
• provide migration notes (that storage_home becomes obsolete, and how to link disks)
• test ceph backend OR mark it as experimental, solution: marked as experimental
• resilient won't be supported, as it makes no sense with external-disk
• review and improve if needed
• finish and merge:
  • https://lab.nexedi.com/nexedi/slapos/merge_requests/1141
  • https://lab.nexedi.com/nexedi/slapos.core/merge_requests/369
• tentative: fail the profile or disallow starting with failing promise if external-disk and external-disk-number are both present