[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/api/authentication/v1/types.go b/vendor/k8s.io/api/authentication/v1/types.go
index d348c6f..c48b036 100644
--- a/vendor/k8s.io/api/authentication/v1/types.go
+++ b/vendor/k8s.io/api/authentication/v1/types.go
@@ -155,7 +155,10 @@
 	ExpirationSeconds *int64 `json:"expirationSeconds" protobuf:"varint,4,opt,name=expirationSeconds"`
 
 	// BoundObjectRef is a reference to an object that the token will be bound to.
-	// The token will only be valid for as long as the bound objet exists.
+	// The token will only be valid for as long as the bound object exists.
+	// NOTE: The API server's TokenReview endpoint will validate the
+	// BoundObjectRef, but other audiences may not. Keep ExpirationSeconds
+	// small if you want prompt revocation.
 	// +optional
 	BoundObjectRef *BoundObjectReference `json:"boundObjectRef" protobuf:"bytes,3,opt,name=boundObjectRef"`
 }