Adding overide for setting etcd version

Added new overide capability to set all etcd version dynamically.  This
is needed for HA testing. For example following flag can now be used with
create cluster.

--override "cluster.spec.etcdClusters*.version=3.0.17"

cmd/kops/create_cluster.go

@@ -1246,6 +1246,10 @@ func setOverrides(overrides []string, cluster *api.Cluster, instanceGroups []*ap
 		switch kv[0] {
 		case "cluster.spec.nodePortAccess":
 			cluster.Spec.NodePortAccess = append(cluster.Spec.NodePortAccess, kv[1])
+		case "cluster.spec.etcdClusters[*].version":
+			for _, etcd := range cluster.Spec.EtcdClusters {
+				etcd.Version = kv[1]
+			}
 		default:
 			return fmt.Errorf("unhandled override: %q", override)
 		}