Make topics configurable for joint controllers

[chapulina]

This should help with #447

Note that I didn't bother using AsValidTopic because that's only useful from Dome.

[codecov]

Codecov Report

Merging #584 (ec90f95) into ign-gazebo3 (4eda53f) will increase coverage by 0.04%.
The diff coverage is 50.00%.

@@               Coverage Diff               @@
##           ign-gazebo3     #584      +/-   ##
===============================================
+ Coverage        77.66%   77.70%   +0.04%     
===============================================
  Files              208      208              
  Lines            11595    11599       +4     
===============================================
+ Hits              9005     9013       +8     
+ Misses            2590     2586       -4     

Impacted Files	Coverage Δ
src/systems/joint_controller/JointController.cc	85.18% <50.00%> (-0.90%)	⬇️
...int_position_controller/JointPositionController.cc	77.55% <50.00%> (-0.58%)	⬇️
src/SimulationRunner.cc	94.53% <0.00%> (+1.12%)	⬆️

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[ahcorde]

@osrf-jenkins retest this please