[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/plugin/pkg/client/auth/exec/exec.go b/vendor/k8s.io/client-go/plugin/pkg/client/auth/exec/exec.go
index 4d72526..b88902c 100644
--- a/vendor/k8s.io/client-go/plugin/pkg/client/auth/exec/exec.go
+++ b/vendor/k8s.io/client-go/plugin/pkg/client/auth/exec/exec.go
@@ -31,8 +31,9 @@
 	"sync"
 	"time"
 
+	"github.com/davecgh/go-spew/spew"
 	"golang.org/x/crypto/ssh/terminal"
-	"k8s.io/apimachinery/pkg/apis/meta/v1"
+	v1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/apimachinery/pkg/runtime/serializer"
@@ -73,8 +74,10 @@
 	return &cache{m: make(map[string]*Authenticator)}
 }
 
+var spewConfig = &spew.ConfigState{DisableMethods: true, Indent: " "}
+
 func cacheKey(c *api.ExecConfig) string {
-	return fmt.Sprintf("%#v", c)
+	return spewConfig.Sprint(c)
 }
 
 type cache struct {
@@ -172,13 +175,9 @@
 // UpdateTransportConfig updates the transport.Config to use credentials
 // returned by the plugin.
 func (a *Authenticator) UpdateTransportConfig(c *transport.Config) error {
-	wt := c.WrapTransport
-	c.WrapTransport = func(rt http.RoundTripper) http.RoundTripper {
-		if wt != nil {
-			rt = wt(rt)
-		}
+	c.Wrap(func(rt http.RoundTripper) http.RoundTripper {
 		return &roundTripper{a, rt}
-	}
+	})
 
 	if c.TLS.GetCert != nil {
 		return errors.New("can't add TLS certificate callback: transport.Config.TLS.GetCert already set")