[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/client_config.go b/vendor/k8s.io/client-go/tools/clientcmd/client_config.go
index dea229c..9c6ac3b 100644
--- a/vendor/k8s.io/client-go/tools/clientcmd/client_config.go
+++ b/vendor/k8s.io/client-go/tools/clientcmd/client_config.go
@@ -228,12 +228,14 @@
 	// blindly overwrite existing values based on precedence
 	if len(configAuthInfo.Token) > 0 {
 		mergedConfig.BearerToken = configAuthInfo.Token
+		mergedConfig.BearerTokenFile = configAuthInfo.TokenFile
 	} else if len(configAuthInfo.TokenFile) > 0 {
-		ts := restclient.NewCachedFileTokenSource(configAuthInfo.TokenFile)
-		if _, err := ts.Token(); err != nil {
+		tokenBytes, err := ioutil.ReadFile(configAuthInfo.TokenFile)
+		if err != nil {
 			return nil, err
 		}
-		mergedConfig.WrapTransport = restclient.TokenSourceWrapTransport(ts)
+		mergedConfig.BearerToken = string(tokenBytes)
+		mergedConfig.BearerTokenFile = configAuthInfo.TokenFile
 	}
 	if len(configAuthInfo.Impersonate) > 0 {
 		mergedConfig.Impersonate = restclient.ImpersonationConfig{
@@ -295,16 +297,6 @@
 	return config
 }
 
-// makeUserIdentificationFieldsConfig returns a client.Config capable of being merged using mergo for only server identification information
-func makeServerIdentificationConfig(info clientauth.Info) restclient.Config {
-	config := restclient.Config{}
-	config.CAFile = info.CAFile
-	if info.Insecure != nil {
-		config.Insecure = *info.Insecure
-	}
-	return config
-}
-
 func canIdentifyUser(config restclient.Config) bool {
 	return len(config.Username) > 0 ||
 		(len(config.CertFile) > 0 || len(config.CertData) > 0) ||
@@ -498,8 +490,9 @@
 		if server := config.overrides.ClusterInfo.Server; len(server) > 0 {
 			icc.Host = server
 		}
-		if token := config.overrides.AuthInfo.Token; len(token) > 0 {
-			icc.BearerToken = token
+		if len(config.overrides.AuthInfo.Token) > 0 || len(config.overrides.AuthInfo.TokenFile) > 0 {
+			icc.BearerToken = config.overrides.AuthInfo.Token
+			icc.BearerTokenFile = config.overrides.AuthInfo.TokenFile
 		}
 		if certificateAuthorityFile := config.overrides.ClusterInfo.CertificateAuthority; len(certificateAuthorityFile) > 0 {
 			icc.TLSClientConfig.CAFile = certificateAuthorityFile