Bump to k8s 1.12.4

See https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG-1.12.md#v1124

Author: mboersma

pkg/api/common/versions.go

@@ -92,6 +92,7 @@ var AllKubernetesSupportedVersions = map[string]bool{
 	"1.12.0":         false,
 	"1.12.1":         false,
 	"1.12.2":         true,
+	"1.12.4":         true,
 	"1.13.0-alpha.1": false,
 	"1.13.0-alpha.2": false,
 	"1.13.1":         true,