fix kubeconfig client merging logic

This change aligns the istioctl client config merging logic with
kubectl. kubectl uses an older version of github.com/imdario/mergo
where the Merge() function overwrites existing values. istioctl uses a
newer version which doesn't (see linked issues below). This manifests
in the k8s client loading code with multiple config sources not being
merged properly.

Longer term fix would be to update kubectl and k8s.io/client-go to use
the latest version of github.com/imdario/mergo.

Fixes istio#4938

Related upstream kubernetes issues:
- darccio/mergo#33 (comment)
- kubernetes/kubernetes#27543
- kubernetes/kubernetes#23789

Gopkg.toml

@@ -361,3 +361,7 @@ ignored = [
 [[constraint]]
   name = "github.com/gogo/status"
   revision = "d60b5acac426cb52f7e28124c6aaa862d9f339f5"
+
+[[override]]
+  name = "github.com/imdario/mergo"
+  revision = "6633656539c1639d9d78127b7d47c622b5d7b6dc"

pkg/kube/config.go

@@ -45,7 +45,11 @@ func BuildClientConfig(kubeconfig, context string) (*rest.Config, error) {
 	loadingRules := clientcmd.NewDefaultClientConfigLoadingRules()
 	loadingRules.DefaultClientConfig = &clientcmd.DefaultClientConfig
 	loadingRules.ExplicitPath = kubeconfig
-	configOverrides := &clientcmd.ConfigOverrides{CurrentContext: context}
+	configOverrides := &clientcmd.ConfigOverrides{
+		ClusterDefaults: clientcmd.ClusterDefaults,
+		CurrentContext:  context,
+	}
+
 	return clientcmd.NewNonInteractiveDeferredLoadingClientConfig(loadingRules, configOverrides).ClientConfig()
 }