[google|compute] Add DiskType resource

- Add models and requests for DiskType
- Add tests
[google|compute] Add support for DiskTypes in Disks

- Add attribute 'type' to Disk model
- Enable 'type' option when inserting a Disk
- Update tests with new 'type' attribute
Merge pull request #3052 from frodenas/gce_disks

[google|compute] Add support for Disk Types