[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/watch/watch.go b/vendor/k8s.io/apimachinery/pkg/watch/watch.go
index be9c90c..3945be3 100644
--- a/vendor/k8s.io/apimachinery/pkg/watch/watch.go
+++ b/vendor/k8s.io/apimachinery/pkg/watch/watch.go
@@ -44,6 +44,7 @@
 	Added    EventType = "ADDED"
 	Modified EventType = "MODIFIED"
 	Deleted  EventType = "DELETED"
+	Bookmark EventType = "BOOKMARK"
 	Error    EventType = "ERROR"
 
 	DefaultChanSize int32 = 100
@@ -57,6 +58,10 @@
 	// Object is:
 	//  * If Type is Added or Modified: the new state of the object.
 	//  * If Type is Deleted: the state of the object immediately before deletion.
+	//  * If Type is Bookmark: the object (instance of a type being watched) where
+	//    only ResourceVersion field is set. On successful restart of watch from a
+	//    bookmark resourceVersion, client is guaranteed to not get repeat event
+	//    nor miss any events.
 	//  * If Type is Error: *api.Status is recommended; other types may make sense
 	//    depending on context.
 	Object runtime.Object