[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/generated.proto b/vendor/k8s.io/api/authentication/v1/generated.proto
index b69636a..db7be17 100644
--- a/vendor/k8s.io/api/authentication/v1/generated.proto
+++ b/vendor/k8s.io/api/authentication/v1/generated.proto
@@ -84,7 +84,10 @@
   optional int64 expirationSeconds = 4;
 
   // 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
   optional BoundObjectReference boundObjectRef = 3;
 }