[google|compute] Improve Disks support

- Add missing "Disk" properties
- Remove unnecessary property aliases
- Use "Operation" instead of the "backoff_if_unfound" method when
  inserting a "Disk" or "Snapshot"
- When destroying a "Disk" asyncronous return the "Operation" model instead
  of the raw operation response
- Add "list_aggregatted_disks" request
- If zone is not set on "all" and "get" methods, use the list aggregated
  disks request (1 API call) instead of fetching all zones
- Fix a bug when inserting a snapshot (description was not merged at the
  body request)
- Fix model tests