Consider switching to snake_case inside XML

[chapulina]

As described on #12 , the SDF parameters being passed to this plugin currently use camelCase, i.e.:

    <plugin filename="libgazebo_ros2_control.so" name="gazebo_ros2_control">
      <robotSimType>gazebo_ros2_control/DefaultRobotHWSim</robotSimType>
      <robotParamNode>robot_state_publisher</robotParamNode>
      <parameters>$(find gazebo_ros2_control_demos)/config/cartpole_controller.yaml</parameters>
    </plugin>

Because both SDF and URDF use snake_case in their specs, when porting gazebo_ros_pkgs to ROS 2, we also changed its XML elements to use that style, see this example.

I recommend using the same here, but feel free to close this issue if you think it's not worth it.

[ahcorde]

PR #24