[JTC] Use the internal methods instead of using the variables directly

[saikishor]

These changes make the JTC use the internal method to retrieve the URDF rather than using the variables directly. I think it is safer and clear this way.

Once the ros-controls/ros2_control#1623 is merged, we would definitely need these changes. These changes can also be merged even without 1623.

Thank you!

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.55%. Comparing base (990bfb6) to head (01fc178).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1221      +/-   ##
==========================================
- Coverage   86.56%   86.55%   -0.02%     
==========================================
  Files          95       95              
  Lines        8585     8586       +1     
  Branches      719      719              
==========================================
  Hits         7432     7432              
  Misses        883      883              
- Partials      270      271       +1     

Flag	Coverage Δ
unittests	86.55% <50.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...ory_controller/src/joint_trajectory_controller.cpp	83.55% <50.00%> (-0.13%)	⬇️

[henrygerardmoore]

Allows ros2_controllers to build with latest ros2_control.