[google|compute] Complete Operations support

- Add missing Operation attributes
- Remove unnecessary attribute aliases
- Add "zone_name" attibute for backward compatibility
- Add "destroy" method for an Operation
- Add "all" method for Operations
- Fix a bug when building an empty response (ie on delete operations)
- Return nil on NotFound Operations (behave like other fog resources)
[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 #2839 from frodenas/gce_operations

[google|compute] Complete Operations support
Merge pull request #2848 from frodenas/gce_snapshots

[google|compute] Modify Snapshots