Updated the launch files.

launch/description.launch

-<!-- -->
+<?xml version="1.0" encoding="UTF-8"?>
 <launch>
 
   <arg name="name" default="model_car"/>

launch/description_test.launch

-<!-- -->
+<?xml version="1.0" encoding="UTF-8"?>
 <launch>
   
   <arg name="name"  default="model_car"/>
+  <arg name="sim_config_path" default="$(find model_car_gazebo)/config"/>
 
   <include file="$(find model_car_description)/launch/description.launch">
     <arg name="name"  value="$(arg name)"/>
+    <arg name="sim_config_path" value="$(arg sim_config_path)"/>
   </include>
-  
-  <node name="joint_state_publisher"
-        pkg ="joint_state_publisher_gui"
-        type="joint_state_publisher_gui">
-    <remap from="/joint_states" 
-             to="/$(arg name)/joint_states" />
-  </node>
+
+  <group ns="$(arg name)">  
+    <node name="joint_state_publisher"
+          pkg ="joint_state_publisher_gui"
+          type="joint_state_publisher_gui">
+      <remap from="/joint_states" 
+               to="/$(arg name)/joint_states" />
+    </node>
+  </group>
 
   <node name="rviz"
         pkg ="rviz"