Make clientcmd defaulting a function of ConfigOverrides and LoadingRules

This commit moves away from using a global variable for default
configuration checking, and instead exposes a method on LoadingRules to
determine whether a particular restclient.Config should be considered
"default". This allows kubectl to provide its own defaults (the same
as before, KUBERNETES_MASTER and the static localhost:8080 values) while
allowing other clients to avoid defining them.

In-cluster config defaulting is now easier to read.

pkg/kubectl/cmd/util/factory.go

@@ -34,7 +34,6 @@ import (
 
 	"github.com/blang/semver"
 	"github.com/emicklei/go-restful/swagger"
-	"github.com/imdario/mergo"
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
 
@@ -1161,11 +1160,13 @@ func (c *clientSwaggerSchema) ValidateBytes(data []byte) error {
 //     exists and is not a directory.
 func DefaultClientConfig(flags *pflag.FlagSet) clientcmd.ClientConfig {
 	loadingRules := clientcmd.NewDefaultClientConfigLoadingRules()
+	// use the standard defaults for this client command
+	// DEPRECATED: remove and replace with something more accurate
+	loadingRules.DefaultClientConfig = &clientcmd.DefaultClientConfig
+
 	flags.StringVar(&loadingRules.ExplicitPath, "kubeconfig", "", "Path to the kubeconfig file to use for CLI requests.")
 
-	overrides := &clientcmd.ConfigOverrides{}
-	// use the standard defaults for this client config
-	mergo.Merge(&overrides.ClusterDefaults, clientcmd.DefaultCluster)
+	overrides := &clientcmd.ConfigOverrides{ClusterDefaults: clientcmd.ClusterDefaults}
 
 	flagNames := clientcmd.RecommendedConfigOverrideFlags("")
 	// short flagnames are disabled by default.  These are here for compatibility with existing scripts