[VOL-1866] Changed module dependency to v12.0.0 of k8s client-go and v1.15.4 of k8s api/apimachinery in sync with other voltha components

Had to use pseudo-version corresponding to v12.0.0 of k8s client-go
because golang proxy is no longer serving the modules not complying
to Semantic Import Versioning rules including client-go v12.0.0.
Refer to https://github.com/kubernetes/client-go/issues/631 and
https://github.com/golang/go/issues/33558

Change-Id: I2e558bab7f0702f230761319eb5392a7d0532ea3
diff --git a/vendor/k8s.io/apimachinery/pkg/api/meta/meta.go b/vendor/k8s.io/apimachinery/pkg/api/meta/meta.go
index b50337e..086bce0 100644
--- a/vendor/k8s.io/apimachinery/pkg/api/meta/meta.go
+++ b/vendor/k8s.io/apimachinery/pkg/api/meta/meta.go
@@ -21,7 +21,6 @@
 	"reflect"
 
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
-	metav1beta1 "k8s.io/apimachinery/pkg/apis/meta/v1beta1"
 	"k8s.io/apimachinery/pkg/conversion"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
@@ -114,12 +113,12 @@
 
 // AsPartialObjectMetadata takes the metav1 interface and returns a partial object.
 // TODO: consider making this solely a conversion action.
-func AsPartialObjectMetadata(m metav1.Object) *metav1beta1.PartialObjectMetadata {
+func AsPartialObjectMetadata(m metav1.Object) *metav1.PartialObjectMetadata {
 	switch t := m.(type) {
 	case *metav1.ObjectMeta:
-		return &metav1beta1.PartialObjectMetadata{ObjectMeta: *t}
+		return &metav1.PartialObjectMetadata{ObjectMeta: *t}
 	default:
-		return &metav1beta1.PartialObjectMetadata{
+		return &metav1.PartialObjectMetadata{
 			ObjectMeta: metav1.ObjectMeta{
 				Name:                       m.GetName(),
 				GenerateName:               m.GetGenerateName(),