[ros2] Port harness to ROS2

[shiveshkhaitan]

Port gazebo_ros_harness plugin to ROS2.

Example:

<plugin name="gazebo_ros_harness" filename="libgazebo_ros_harness.so">
  <ros>
    <!-- Add a namespace -->
    <namespace>demo</namespace>
    <argument>box/harness/velocity:=velocity_demo</argument>
    <argument>box/harness/detach:=detach_demo</argument>
  </ros>
  <!-- set initial harness velocity -->
  <init_vel>-0.1</init_vel>
</plugin>

Migration guide at ROS 2 Migration: Harness

[chapulina]

@osrf-jenkins run tests please

[chapulina]

Nice, I like the new init vel feature and the change from Bool to Empty!

The test passes for me and the OSRF farm, but it fails on the ROS farm with the same problem the elevator is having: libHarnessPlugin.so: cannot open shared object file: No such file or directory.

I ticketed #970 for this. Since it doesn't seem to be an issue specific to this PR, I'll merge this as soon as it's rebased.