[PidROS] Enable interpreting prefix as param prefix.

[destogl]

When using PIDController (ros-controls/ros2_controllers#434) as preceeding controller we should not remove "/" from the parameter prefix since this causes a confusion between parameters and how we are defining reference interfaces ("controller_name/interface") in ros2_control.

This enalbes using of parameters prefixes for following interfaces, e.g.:

pid_controller:
  ros__parameters:

    dof_names:
      - following_controller/joint28
      - following_controller/joint23

    ...

    gains:
      following_controller/joint28: {"p": 28.0, "i": 28.0, ...}
      following_controller/joint23: {"p": 23.0, "i": 23.0, ...}

[christophfroehlich]

never used the namespacing stuff to give proper feedback, but the changes look good to me

[destogl]

@bmagyar can we please go on with this?

[christophfroehlich]

only minor changes to doc

[bmagyar]

Also created a follow-up #156