Use highest available magnum microversion

Magnum allows using the microversion string "latest",
and it will replace it internally with the highest
microversion that it supports.

This will let the autoscaler use microversion 1.10 which
allows scaling groups to 0 nodes, if it is available.

The autoscaler will still be able to use microversion 1.9
on older versions of magnum.

cluster-autoscaler/cloudprovider/magnum/README.md

@@ -16,6 +16,8 @@ using the cluster autoscaler v1.18 or lower.
 
 ## Updates
 
+* CA 1.22
+  * Allow scaling node groups to 0 nodes, if supported (requires Magnum Wallaby).
 * CA 1.19
   * Update to support Magnum node groups (introduced in Magnum Train).
     * Add node group autodiscovery based on the group's role property.

cluster-autoscaler/cloudprovider/magnum/magnum_manager.go

@@ -29,6 +29,10 @@ import (
 const (
 	// Magnum microversion that must be requested to use the node groups API.
 	microversionNodeGroups = "1.9"
+	// Magnum microversion that must be requested to support scaling node groups to 0 nodes.
+	microversionScaleToZero = "1.10"
+	// Magnum interprets "latest" to mean the highest available microversion.
+	microversionLatest = "latest"
 )
 
 // magnumManager is an interface for the basic interactions with the cluster.
@@ -61,7 +65,7 @@ func createMagnumManager(configReader io.Reader, discoverOpts cloudprovider.Node
 		return nil, err
 	}
 
-	clusterClient.Microversion = microversionNodeGroups
+	clusterClient.Microversion = microversionLatest
 
 	// This replaces the cluster name with a UUID if the name was given in the parameters.
 	err = checkClusterUUID(provider, clusterClient, opts)