Add defaults subcommand to `minikube config`

`minikube config defaults PROPERTY_NAME` will print out all acceptable default values for that property.

Currently only works for `minikube config defaults driver` which looks like this on Mac:

```
$ minikube config defaults driver
* virtualbox
* parallels
* vmwarefusion
* hyperkit
* vmware
* docker
* podman
```

This will be useful for clients that depend on minikube, like IDE extensions.

cmd/minikube/cmd/config/config.go

@@ -22,6 +22,7 @@ import (
 	"github.com/golang/glog"
 	"github.com/spf13/cobra"
 	"k8s.io/minikube/pkg/minikube/config"
+	"k8s.io/minikube/pkg/minikube/driver"
 	"k8s.io/minikube/pkg/minikube/localpath"
 )
 
@@ -32,21 +33,23 @@ type setFn func(string, string) error
 
 // Setting represents a setting
 type Setting struct {
-	name        string
-	set         func(config.MinikubeConfig, string, string) error
-	setMap      func(config.MinikubeConfig, string, map[string]interface{}) error
-	validations []setFn
-	callbacks   []setFn
+	name          string
+	set           func(config.MinikubeConfig, string, string) error
+	setMap        func(config.MinikubeConfig, string, map[string]interface{}) error
+	validDefaults func() []string
+	validations   []setFn
+	callbacks     []setFn
 }
 
 // These are all the settings that are configurable
 // and their validation and callback fn run on Set
 var settings = []Setting{
 	{
-		name:        "driver",
-		set:         SetString,
-		validations: []setFn{IsValidDriver},
-		callbacks:   []setFn{RequiresRestartMsg},
+		name:          "driver",
+		set:           SetString,
+		validDefaults: driver.SupportedDrivers,
+		validations:   []setFn{IsValidDriver},
+		callbacks:     []setFn{RequiresRestartMsg},
 	},
 	{
 		name:        "vm-driver",

cmd/minikube/cmd/config/defaults.go

@@ -0,0 +1,74 @@
+/*
+Copyright 2016 The Kubernetes Authors All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package config
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/spf13/cobra"
+)
+
+var configDefaultsCommand = &cobra.Command{
+	Use:   "defaults PROPERTY_NAME",
+	Short: "Lists all valid default values for PROPERTY_NAME",
+	Long: `list displays all valid default settings for PROPERTY_NAME
+Acceptable fields: ` + "\n\n" + fieldsWithDefaults(),
+	RunE: func(cmd *cobra.Command, args []string) error {
+		if len(args) == 0 {
+			cmd.SilenceErrors = true
+			return errors.New("not enough arguments.\nusage: minikube config list PROPERTY_NAME")
+		}
+		if len(args) > 1 {
+			cmd.SilenceErrors = true
+			return fmt.Errorf("too many arguments (%d)\nusage: minikube config list PROPERTY_NAME", len(args))
+		}
+
+		property := args[0]
+		return listDefaults(property)
+	},
+}
+
+func listDefaults(property string) error {
+	setting, err := findSetting(property)
+	if err != nil {
+		return err
+	}
+	if setting.validDefaults == nil {
+		return fmt.Errorf("%s is not a valid option for the `defaults` command; to see valid options run `minikube config defaults -h`", property)
+	}
+	defaults := setting.validDefaults()
+	for _, d := range defaults {
+		fmt.Printf("* %s\n", d)
+	}
+	return nil
+}
+
+func fieldsWithDefaults() string {
+	fields := []string{}
+	for _, s := range settings {
+		if s.validDefaults != nil {
+			fields = append(fields, " * "+s.name)
+		}
+	}
+	return strings.Join(fields, "\n")
+}
+
+func init() {
+	ConfigCmd.AddCommand(configDefaultsCommand)
+}