Added param robot name in the .ini files in order to use the singleton.

Added param robot name in the .ini files in order to use the singleton inside yarp plugins.

This singleton will store the gazebo::physic::model pointer of the robot and give it to the plugin who need it.
The singleton can happily handle more than one robot.
The match is done by looking at the 'robot' keyword in the .ini file and the 'model' name in the .sdf file imported
by gazebo.