Refactor to distinguish between wheel and steering joints (steering controller)

Pre-requirement of #648

If you have a look for example at:
https://github.com/ros-controls/ros2_controllers/blob/master/ackermann_steering_controller/test/ackermann_steering_controller_params.yaml
you can see that front_wheels_names and rear_wheels_names in fact might refer (depending on situation) to either a traction actuator or a steering actuator.

Here:
https://github.com/ros-controls/ros2_controllers/blob/76b1ce7da5e7a102ef3a1c8c4f1b0de90ed0d137/steering_controllers_library/src/steering_controllers_library.cpp#L280-L316
a distinction is then made depending on front_steering param (and linked to #648, I should introduce here somehow all_wheel_drive parameter).

It feels to me that otherwise introducing additional implicit conventions in combination of new flags / number of passed joints and their interpretation as wheels or steers will complicate even more the steering_controller package...

Semantically, we have no enforcement at configuration level of what is "wheel" and what is "steer". I propose to add front_steer_names and rear_steer_names, and refactor the code in support of both #648 and #526 (which I am also involved in).

Start with steering_controllers_library/src/steering_controllers_library.cpp around lines 280-316 and the example ackermann_steering_controller/test/ackermann_steering_controller_params.yaml. Trace how front_wheels_names and rear_wheels_names are interpreted with front_steering, then review the requirements in #648 and #526. Done means wheel and steering joints are explicitly distinguishable through the proposed front_steer_names and rear_steer_names configuration.