allow to start gazebo without gui

ur_gazebo/launch/ur10.launch

@@ -2,11 +2,13 @@
 <launch>
   <arg name="limited" default="false"/>
   <arg name="paused" default="false"/>
-
+  <arg name="gui" default="true"/>
+  
   <!-- startup simulated world -->
   <include file="$(find gazebo_ros)/launch/empty_world.launch">
     <arg name="world_name" default="worlds/empty.world"/>
     <arg name="paused" value="$(arg paused)"/>
+    <arg name="gui" value="$(arg gui)"/>
   </include>
 
   <!-- send robot urdf to param server -->

ur_gazebo/launch/ur10_joint_limited.launch

 <?xml version="1.0"?>
 <launch>
+  <arg name="gui" default="true"/>
 
   <include file="$(find ur_gazebo)/launch/ur10.launch">
     <arg name="limited" value="true"/>
+    <arg name="gui" value="$(arg gui)"/>
   </include>
 
 </launch>

ur_gazebo/launch/ur5.launch

@@ -2,11 +2,13 @@
 <launch>
   <arg name="limited" default="false"/>
   <arg name="paused" default="false"/>
+  <arg name="gui" default="true"/>
   
   <!-- startup simulated world -->
   <include file="$(find gazebo_ros)/launch/empty_world.launch">
     <arg name="world_name" default="worlds/empty.world"/>
     <arg name="paused" value="$(arg paused)"/>
+    <arg name="gui" value="$(arg gui)"/>
   </include>
 
   <!-- send robot urdf to param server -->

ur_gazebo/launch/ur5_joint_limited.launch

 <?xml version="1.0"?>
 <launch>
+  <arg name="gui" default="true"/>
 
   <include file="$(find ur_gazebo)/launch/ur5.launch">
     <arg name="limited" value="true"/>
+    <arg name="gui" value="$(arg gui)"/>
   </include>
 
 </launch>