gazebo_plugins: moveit planning scene publisher

This gazebo plugin publishes all of the objects in a gazebo world to a moveit planning scene topic. It can either publish at a given rate, or be polled manually.

Example usage:
```xml
      <gazebo>
        <plugin filename="libgazebo_ros_moveit_planning_scene.so" name="gazebo_ros_moveit_planning_scene">
          <topicName>/planning_scene</topicName>
          <sceneName>laboratory</sceneName>
          <robotName>my_robot</robotName>
          <updatePeriod>0.5</updatePeriod>
        </plugin>
      </gazebo>

```

- [ ] subscribe to published planning scene in order to make sure that models have already been added

Shoutout @davetcoleman @isucan
Can I go ahead and merge this pull request without this modification, and propose adding back this change in a separate pull request?