provider/gce: Add a zoneName helper.
Pull out the inlined newInstance call.
Move a log message up.
provider/gce: Add some disk-related helpers.
provider/gce: Add a stub for getting disk info from GCE.
provider/gce: Call inst.update in newInstance.
provider/gce: Drop the getInstance method.
provider/gce: Drop the env parameter.
provider/gce: Store the root disk size on the instance.