added paramter for loading gazebo ros control plugin for multiple robot set up

[bi3ri]

If I want to create a multiple robot setup and simulate it via gazebo I only want to start one gazebo ros control plugin, because two will lead to conflicts.

With this PR it is possible to start the plugin with just one robot and set the parameter "load_gazebo_ros_control_plugin" for others to false.

Before there was a common.gazebo.xacro file, which could be loaded just once for a setup.