This was done as its own resource as suggested in slack, since we don't have the option of making all fields Computed in google_compute_instance. There's precedent in the aws provider for this sort of thing (see ami_copy, ami_from_instance).
When I started working on this I assumed I could do it in the compute_instance resource and so I went ahead and reordered the schema to make it easier to work with in the future. Now it's not quite relevant, but I left it in as its own commit that can be looked at separately from the other changes.
Fixes#1582.
