[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/tools/clientcmd/loader.go b/vendor/k8s.io/client-go/tools/clientcmd/loader.go
index 7e928a9..e00ea38 100644
--- a/vendor/k8s.io/client-go/tools/clientcmd/loader.go
+++ b/vendor/k8s.io/client-go/tools/clientcmd/loader.go
@@ -356,7 +356,7 @@
 	if err != nil {
 		return nil, err
 	}
-	klog.V(6).Infoln("Config loaded from file", filename)
+	klog.V(6).Infoln("Config loaded from file: ", filename)
 
 	// set LocationOfOrigin on every Cluster, User, and Context
 	for key, obj := range config.AuthInfos {