better 'zrok config unset' supporting defaultFrontend; also properly extensible (#663)

cmd/zrok/configUnset.go

@@ -3,7 +3,6 @@ package main
 import (
 	"fmt"
 	"github.com/openziti/zrok/environment"
-	"github.com/openziti/zrok/environment/env_core"
 	"github.com/openziti/zrok/tui"
 	"github.com/spf13/cobra"
 	"os"
@@ -36,18 +35,25 @@ func (cmd *configUnsetCommand) run(_ *cobra.Command, args []string) {
 		panic(err)
 	}
 
+	if env.Config() != nil {
+		cfg := env.Config()
 		switch configName {
 		case "apiEndpoint":
-		if err := env.SetConfig(&env_core.Config{}); err != nil {
+			cfg.ApiEndpoint = ""
-			tui.Error("unable to save config", err)
-		}
-		fmt.Println("zrok configuration updated")
 			if env.IsEnabled() {
 				fmt.Printf("\n[%v]: because you have a %v-d environment, you won't see your config change until you run %v first!\n\n", tui.WarningLabel, tui.Code.Render("zrok enable"), tui.Code.Render("zrok disable"))
 			}
 
+		case "defaultFrontend":
+			cfg.DefaultFrontend = ""
+			
 		default:
 			fmt.Printf("unknown config name '%v'\n", configName)
 			os.Exit(1)
 		}
+		if err := env.SetConfig(cfg); err != nil {
+			tui.Error("unable to save config", err)
+		}
+		fmt.Println("zrok configuration updated")
+	}
 }