make diff drive load correctly

[Karsten1987]

fixes #70 (comment)

The launch file got also enhanced to load and start the joint state controller directly. Starting a Joint State Publisher doesn't make much sense in my opinion if you want to load the joint state controller at the same time.

[codecov-commenter]

Codecov Report

Merging #89 (13214ee) into master (1e14150) will not change coverage.
The diff coverage is n/a.

❗ Current head 13214ee differs from pull request most recent head 5d2a796. Consider uploading reports for the commit 5d2a796 to get more accurate results

@@           Coverage Diff           @@
##           master     #89    +/-   ##
=======================================
  Coverage    0.00%   0.00%            
=======================================
  Files           9       3     -6     
  Lines         849     283   -566     
=======================================
+ Misses        849     283   -566     

Flag	Coverage Δ
unittests	0.00% <ø> (ø)

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Impacted Files	Coverage Δ
.../ros2_control_demo_hardware/src/diffbot_system.cpp
...demo_hardware/src/rrbot_system_multi_interface.cpp
...l_demo_hardware/src/rrbot_system_position_only.cpp
...demo_hardware/src/rrbot_system_multi_interface.cpp
...l_demo_hardware/src/rrbot_system_position_only.cpp
...l_demo_hardware/src/rrbot_system_position_only.cpp
...demo_hardware/src/rrbot_system_multi_interface.cpp
.../ros2_control_demo_hardware/src/diffbot_system.cpp
.../ros2_control_demo_hardware/src/diffbot_system.cpp
...l_demo_hardware/src/rrbot_system_position_only.cpp	0.00% <0.00%> (ø)
... and 2 more

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[bmagyar]

I've fixed the cpplint warnings + ran precommit