Ubernetes-Lite: Error if a PD name is ambiguous

We don't cope well if a PD is in multiple zones, but this is actually
fairly easy to detect.  This is probably justified purely on the basis
that we never want to delete the wrong volume (DeleteDisk), but also
because this means that we now warn on creation if a disk is in multiple
zones (with the labeling admission controller).

This also means that with the scheduling predicate in place, that many
of our volume problems "go away" in practice: you still can't create or
delete a volume when it is ambiguous, but thereafter the volume will be
labeled with the zone, that will match it only to nodes with the same
zone, and then we query for the volume in that zone when we
attach/detach it.

pkg/cloudprovider/providers/gce/gce.go

@@ -1764,14 +1764,20 @@ func (gce *GCECloud) getDiskByNameUnknownZone(diskName string) (*gceDisk, error)
 	// "us-central1-a/mydisk".  We could do this for them as part of
 	// admission control, but that might be a little weird (values changing
 	// on create)
+
+	var found *gceDisk
 	for _, zone := range gce.managedZones {
 		disk, err := gce.findDiskByName(diskName, zone)
 		if err != nil {
 			return nil, err
 		}
-		if disk != nil {
-			return disk, nil
+		if found != nil {
+			return nil, fmt.Errorf("GCE persistent disk name was found in multiple zones: %q", diskName)
 		}
+		found = disk
+	}
+	if found != nil {
+		return found, nil
 	}
 	return nil, fmt.Errorf("GCE persistent disk not found: %q", diskName)
 }