etcdctl: print messages for successful auth operations

This commit lets etcdctl v3 follow the manner of etcdctl v2.

etcdctl/ctlv3/command/auth_command.go

@@ -52,4 +52,6 @@ func authEnableCommandFunc(cmd *cobra.Command, args []string) {
 	if err != nil {
 		ExitWithError(ExitError, err)
 	}
+
+	fmt.Println("Authentication Enabled")
 }

etcdctl/ctlv3/command/user_command.go

@@ -97,6 +97,8 @@ func userAddCommandFunc(cmd *cobra.Command, args []string) {
 	if err != nil {
 		ExitWithError(ExitError, err)
 	}
+
+	fmt.Printf("User %s created\n", args[0])
 }
 
 // userDeleteCommandFunc executes the "user delete" command.
@@ -109,4 +111,6 @@ func userDeleteCommandFunc(cmd *cobra.Command, args []string) {
 	if err != nil {
 		ExitWithError(ExitError, err)
 	}
+
+	fmt.Printf("User %s deleted\n", args[0])
 }