ROS plugin - SDF empty robotNamespace

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I've built up a robot model robotino.sdf(see below) which uses the slightly modified myhokuyo.sdf laser scanner model (see below, just added the ros-laser plugin).

Now if I start gazebo with roslaunch gazebo_ros empty_world.launch and add the robot model, I get this output:

[ INFO] [1424682979.068295221, 7.402000000]: Laser Plugin (robotNamespace = ), Info: the 'robotNamespace' param did not exit
[ INFO] [1424682979.068369479, 7.402000000]: Starting Laser Plugin (ns = )!
[ INFO] [1424682979.069371065, 7.403000000]: Laser Plugin (ns = )  <tf_prefix_>, set to ""

rostopic list outputs:

/clock
/gazebo/link_states
/gazebo/model_states
/gazebo/parameter_descriptions
/gazebo/parameter_updates
/gazebo/set_link_state
/gazebo/set_model_state
/laser_scan
/rosout
/rosout_agg

How can I define the robotNamespace within robotino.sdf so that the laser_scan topic is published under /robotino_bbu/laser_scan?

I need different topics since there will be multiple instances of the same model in one simulation.

robotino.sdf file:

<?xml version="1.0" ?>
<sdf version="1.4">

    <model name="robotino_bbu">

    <include>
      <uri>model://robotino3</uri>
      <pose>0 0 0 0 0 0</pose>
    </include>


    <include>
      <uri>model://myhokuyo</uri>
      <pose>0.0 -0.15 0.235 0 0 -1.570796</pose>
    </include>

    <joint name="hokuyo_joint" type="revolute">
      <parent>robotino3::body</parent>
      <child>myhokuyo::link</child>
      <axis>
        <xyz>0 0 1</xyz>
        <limit>
          <upper>0</upper>
          <lower>0</lower>
        </limit>
      </axis>
    </joint>

    <plugin name="Odometry" filename="libodometry.so"/>

  </model>
</sdf>

myhokuyo.sdf

<?xml version="1.0" ?>
<sdf version="1.4">
  <model name="myhokuyo">
    <pose>0 0 0.035 0 0 0</pose>
    <link name="link">
      <inertial>
        <mass>0.1</mass>
      </inertial>
      <visual name="visual">
        <geometry>
          <mesh>
            <uri>model://hokuyo/meshes/hokuyo.dae</uri>
          </mesh>
        </geometry>
      </visual>

      <collision name="collision-base">
        <pose>0 0 -0.0145 0 0 0</pose>
        <geometry>
          <box>
            <size>0.05 0.05 0.041</size>
          </box>
        </geometry>
      </collision>

      <collision name="collision-top">
        <pose>0 0 0.0205 0 0 0</pose>
        <geometry>
          <cylinder>
            <radius>0.021</radius>
            <length>0.029</length>
          </cylinder>
        </geometry>
      </collision>


      <sensor name="laser" type="ray">
        <pose>0.01 0 0.0175 0 -0 0</pose>
        <ray>
          <scan>
            <horizontal>
              <samples>640</samples>
              <resolution>1</resolution>
              <min_angle>-2.26889</min_angle>
              <max_angle>2.268899</max_angle>
            </horizontal>
          </scan>
          <range>
            <min>0.08</min>
            <max>10</max>
            <resolution>0.01</resolution>
          </range>
        </ray>

        <plugin name="laser" filename="libRayPlugin.so" />
    <plugin name="link_laser_controller" filename="libgazebo_ros_laser.so">
            <topicName>laser_scan</topicName>
            <frameName>link</frameName>
    </plugin>
        <always_on>1</always_on>
        <update_rate>30</update_rate>
        <visualize>true</visualize>
      </sensor>
    </link>
  </model>
</sdf>