auth: Adding support for "auth disable" command.

Added support for the auth disable command in the server, added the
etcdctl command and a respective testcase.

etcdctl/ctlv3/command/auth_command.go

@@ -28,6 +28,7 @@ func NewAuthCommand() *cobra.Command {
 	}
 
 	ac.AddCommand(newAuthEnableCommand())
+	ac.AddCommand(newAuthDisableCommand())
 
 	return ac
 }
@@ -43,7 +44,7 @@ func newAuthEnableCommand() *cobra.Command {
 // authEnableCommandFunc executes the "auth enable" command.
 func authEnableCommandFunc(cmd *cobra.Command, args []string) {
 	if len(args) != 0 {
-		ExitWithError(ExitBadArgs, fmt.Errorf("auth enable command does not accept argument."))
+		ExitWithError(ExitBadArgs, fmt.Errorf("auth enable command does not accept any arguments."))
 	}
 
 	ctx, cancel := commandCtx(cmd)
@@ -55,3 +56,27 @@ func authEnableCommandFunc(cmd *cobra.Command, args []string) {
 
 	fmt.Println("Authentication Enabled")
 }
+
+func newAuthDisableCommand() *cobra.Command {
+	return &cobra.Command{
+		Use:   "disable",
+		Short: "disable authentication",
+		Run:   authDisableCommandFunc,
+	}
+}
+
+// authDisableCommandFunc executes the "auth disable" command.
+func authDisableCommandFunc(cmd *cobra.Command, args []string) {
+	if len(args) != 0 {
+		ExitWithError(ExitBadArgs, fmt.Errorf("auth disable command does not accept any arguments."))
+	}
+
+	ctx, cancel := commandCtx(cmd)
+	_, err := mustClientFromCmd(cmd).Auth.AuthDisable(ctx)
+	cancel()
+	if err != nil {
+		ExitWithError(ExitError, err)
+	}
+
+	fmt.Println("Authentication Disabled")
+}