provider/gce: Use a minimum root disk size for new instances.
provider/gce: Add a TODO about deleting disks.
provider/gce: Drop another TODO.
provider/gce: Handle errors better (try next zone) in newInstance.
provider/gce: newInstance -> addInstance.
Drop a TODO.
provider/gce: Address some TODOs related to disks.
provider/gce: projectId -> projectID