[google|compute] When 'insert'ing a disk, don't try to create an image.

This patch avoids trying to create an image when trying to insert a disk (because we don't have sufficient information to create an image at that point).

I'll fix up the ```insert_disk``` in a subsequent patch (align it better with the [API](https://developers.google.com/compute/docs/reference/latest/disks/insert))