[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/client-go/util/flowcontrol/backoff.go b/vendor/k8s.io/client-go/util/flowcontrol/backoff.go
index 71d442a..39cd72f 100644
--- a/vendor/k8s.io/client-go/util/flowcontrol/backoff.go
+++ b/vendor/k8s.io/client-go/util/flowcontrol/backoff.go
@@ -21,7 +21,7 @@
 	"time"
 
 	"k8s.io/apimachinery/pkg/util/clock"
-	"k8s.io/client-go/util/integer"
+	"k8s.io/utils/integer"
 )
 
 type backoffEntry struct {
@@ -99,7 +99,7 @@
 	if hasExpired(eventTime, entry.lastUpdate, p.maxDuration) {
 		return false
 	}
-	return p.Clock.Now().Sub(eventTime) < entry.backoff
+	return p.Clock.Since(eventTime) < entry.backoff
 }
 
 // Returns True if time since lastupdate is less than the current backoff window.