Update the Google Compute Engine provider to add support for service
accounts on `google_compute_instance`. Both gcloud shorthand (`compute-ro`,
`storage-ro`, etc.) and OAuth2 API endpoints are supported.
This feature is currently limited to a single service account (supporting
multiple scopes) and an automatically-generated service account email.
Supports configuring the disk type for Google Compute Engine disk
resources. Both `google_compute_disk` and `google_compute_instance`
disk types are supported.
Resolves#351.
Emil Hessman