[google|compute] Modify Snapshots

- Add missing "Snapshot" properties
- Fix "delete" request, as "Snapshots" doesn't require a zone
- Added "destroy" and "ready?" methods to model
- Removed "project" param
- Removed custom "reload" method in favour of the standard fog method
Merge pull request #2848 from frodenas/gce_snapshots

[google|compute] Modify Snapshots