UPSTREAM: <carry>: Compare against minSize in deleteNodes()

When calling deleteNodes() we should fail early if the operation could delete nodes below the nodeGroup minSize().

This is one in a series of PR to mitigate kubernetes#3104
Once we got all merge we'll put a PR upstream.

cluster-autoscaler/cloudprovider/clusterapi/clusterapi_nodegroup.go

@@ -92,6 +92,7 @@ func (ng *nodegroup) IncreaseSize(delta int) error {
 // Implementation required.
 func (ng *nodegroup) DeleteNodes(nodes []*corev1.Node) error {
 	// Step 1: Verify all nodes belong to this node group.
+
 	for _, node := range nodes {
 		actualNodeGroup, err := ng.machineController.nodeGroupForNode(node)
 		if err != nil {
@@ -112,10 +113,8 @@ func (ng *nodegroup) DeleteNodes(nodes []*corev1.Node) error {
 	// and we fail fast.
 	replicas := ng.scalableResource.Replicas()
 
-	if replicas-int32(len(nodes)) < 0 {
-		return fmt.Errorf("unable to delete %d machines in %q, machine replicas are < 0 ", len(nodes), ng.Id())
-	} else if replicas-int32(len(nodes)) == 0 && !ng.scalableResource.CanScaleFromZero() {
-		return fmt.Errorf("unable to delete %d machines in %q, machine replicas are 0", len(nodes), ng.Id())
+	if replicas-int32(len(nodes)) <= int32(ng.MinSize()) {
+		return fmt.Errorf("unable to delete %d machines in %q, machine replicas are <= %d", len(nodes), ng.Id(), ng.MinSize())
 	}
 
 	// Step 3: annotate the corresponding machine that it is a