[ros2] Spawn <plugin> without <ros>

[chapulina]

The issue was brought up by @mogumbo here: #948 (comment) .

When traversing SDF elements, we need to add element descriptions for each element, otherwise they're not created. This is usually not an issue because SDF can tell we're dealing with a <plugin> tag, which accepts arbitrary elements. But in this use case, we're dealing with generic SDF elements, so we need to describe the contents.

A manual way of testing this is to create a URDF model like this:

<?xml version="1.0" ?>
<robot name="ball">
<link name="base_link">
<visual>
<geometry>
<sphere radius="1.0"/>
</geometry>
</visual>
<collision>
<geometry>
<sphere radius="1.0"/>
</geometry>
</collision>
<inertial>
<mass value="1"/>
<inertia ixx="1" ixy="0.0" ixz="0.0" iyy="1" iyz="0.0" izz="1"/>
</inertial>
</link>
<gazebo>
    <plugin filename="libgazebo_ros_p3d.so" name="gazebo_ros_p3d">
        <!--ros>
          <argument>odom:=ground_truth</argument>
        </ros-->
        <frame_name>world</frame_name>
        <body_name>base_link</body_name>
        <update_rate>1</update_rate>
    </plugin>
  </gazebo>
</robot>

And spawn it like this:

ros2 run gazebo_ros spawn_entity.py -entity ball -file /path/to/model.urdf

[jacobperron]

LGTM (with my limited knowledge of gazebo_ros_pkgs 😁)