Implement fmt.Stringer on rest.Config to sanitize sensitive fields

It's very easy to add glog.Info(config) calls for debugging (or actual
logging). In some scenarios those configs will carry sensitive tokens
and those tokens will end up in logs or response bodies.
Leaking of those stringified configs compromises the cluster.

Also implement fmt.GoStringer.

Kubernetes-commit: c9ad1d7339b164dfba0846ec49fa4a52474d3e23

plugin/pkg/client/auth/exec/exec.go

@@ -31,6 +31,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/davecgh/go-spew/spew"
 	"golang.org/x/crypto/ssh/terminal"
 	v1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/runtime"
@@ -73,8 +74,10 @@ func newCache() *cache {
 	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 {