Two individual Gazebo Fetch tests for Physical Virtual Case

[xiangfeikong777]

Relevant issue

closes #N No related issue currently.

Describe/summarize the changes

1. So far, I am still working on setting another individual fetch in the Gazebo to test the physical, virtual case.
   I will work with namespace=fetch2 and robot_name=virtual_robot as the virtual fetch.
   The default robot in Gazebo is the physical fetch for the test later.

Show the changes work or is high-quality

Set the namespace to fetch2 for all nodes and topics related to the robot.
Pass virtual_robot as the robot name to the included launch files.

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• If applicable, revise or add documentation (readme, code comments) so others can understand and reproduce the changes. If docs are added, describe in "Summarize the changes" above.
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[xiangfeikong777]

Update:

1. The gazebo_simulation_scene_fetch.launch file can launched but without fetch.
2. The output shows me the active ROS nodes and topics after launching the Gazebo simulation with the fetch2 namespace and virtual_robot. The default nodes and topics separate them.