Parametrize sensor description when using standard laser instead of gpu laser

[fjp]

On some platforms it is necessary to use the standard/cpu gazebo laser plugin instead of the gazebo gpu laser plugin (e.g. without dedicated gpu). Make it possible to switch between gazebo laser plugin and the gazebo gpu laser plugin and adjust the parameters (update rate, samples, ..) to avoid ghost objects:

See the description of turtlebot3 to avoid those issues.

Make sure to set reasonable range values to avoid hitting the robot model itself:

<range>
    <min>0.12</min>
    <max>12.0</max>
    <resolution>0.01</resolution>
  </range>

Update documentation:

• http://gazebosim.org/tutorials?tut=sensor_noise
• http://gazebosim.org/tutorials?tut=ros_gzplugins#GPULaser
• http://gazebosim.org/tutorials?tut=ros_gzplugins#Laser