Confuse about env name and env loader

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Here is my launch file:

<launch>
  <group ns="rover1">
    <env name="ROS_MASTER_URI" value="http://Master:11311"/> 
    <machine name="Slaver1" address="Slaver1" user="pi"  env-loader="/opt/ros/indigo/env_rover.sh" />
    <machine name="Master"  address="Master"  user="hlr" env-loader="/opt/ros/indigo/env_rover.sh" /> 
    <node pkg="rover_net" name="ControlNode" type="ControlNode" />
    <node machine="Slaver1" pkg="rover_net" name="SensorNode" type="SensorNode" />
    <node machine="Slaver1" pkg="rosserial_python" name="serial_node" type="serial_node.py" args="/dev/ttyACM0" />
  </group>
</launch>

AND below is my env-loader file:

#!/bin/sh
. /home/pi/catkin_ws/devel/setup.sh
exec "$@"

My qustion is since I have set the ROS_MASTER_URI but the remote machine is also:

Slaver1-0]: ROS_MASTER_URI=http://localhost:1131

unless I add:

export ROS_MASTER_URI=http://Master:11311

It seems that the env name have no work. I am confuse about How the args we set in the launch file pass to the env_rover.sh? Thank for Anything Help!