fixed launchfile to properly support a robot name

minibot_vision_workshops/launch/traffic_sign_workshop.launch

 <?xml version="1.0"?>
 <launch>
-    <group ns="$(env ROBOT)" >
+    <arg name="robot_name" default="$(optenv ROBOT minibot)" />
+
+    <group ns="$(arg robot_name)" >
         <include file="$(find minibot_vision)/launch/sign_classifier.launch" >
             <arg name="classification_fps" value="3" />
             <arg name="visualization_compression_factor" value="1.0" />