[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/labels/labels.go b/vendor/k8s.io/apimachinery/pkg/labels/labels.go
index 32db4d9..abf3ace 100644
--- a/vendor/k8s.io/apimachinery/pkg/labels/labels.go
+++ b/vendor/k8s.io/apimachinery/pkg/labels/labels.go
@@ -172,7 +172,7 @@
 			return labelsMap, err
 		}
 		value := strings.TrimSpace(l[1])
-		if err := validateLabelValue(value); err != nil {
+		if err := validateLabelValue(key, value); err != nil {
 			return labelsMap, err
 		}
 		labelsMap[key] = value
diff --git a/vendor/k8s.io/apimachinery/pkg/labels/selector.go b/vendor/k8s.io/apimachinery/pkg/labels/selector.go
index f5a0888..9be9e57 100644
--- a/vendor/k8s.io/apimachinery/pkg/labels/selector.go
+++ b/vendor/k8s.io/apimachinery/pkg/labels/selector.go
@@ -162,7 +162,7 @@
 	}
 
 	for i := range vals {
-		if err := validateLabelValue(vals[i]); err != nil {
+		if err := validateLabelValue(key, vals[i]); err != nil {
 			return nil, err
 		}
 	}
@@ -837,9 +837,9 @@
 	return nil
 }
 
-func validateLabelValue(v string) error {
+func validateLabelValue(k, v string) error {
 	if errs := validation.IsValidLabelValue(v); len(errs) != 0 {
-		return fmt.Errorf("invalid label value: %q: %s", v, strings.Join(errs, "; "))
+		return fmt.Errorf("invalid label value: %q: at key: %q: %s", v, k, strings.Join(errs, "; "))
 	}
 	return nil
 }