Fix unload of controllers when spawned with --unload-on-kill

[saikishor]

Fix the bug, when using --unload-on-kill option, the spawner only unloads the last controller instead of all the controllers. This PR proposes a solution to fix the above issue

[codecov]

Codecov Report

Attention: Patch coverage is 70.58824% with 5 lines in your changes missing coverage. Please review.

Project coverage is 86.75%. Comparing base (6164883) to head (373dd13).
Report is 2 commits behind head on master.

Files	Patch %	Lines
controller_manager/controller_manager/spawner.py	50.00%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1717      +/-   ##
==========================================
+ Coverage   86.73%   86.75%   +0.02%     
==========================================
  Files         116      116              
  Lines       10618    10628      +10     
  Branches      978      980       +2     
==========================================
+ Hits         9209     9220      +11     
+ Misses       1058     1057       -1     
  Partials      351      351              

Flag	Coverage Δ
unittests	86.75% <70.58%> (+0.02%)	⬆️

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

Files	Coverage Δ
controller_manager/test/test_spawner_unspawner.cpp	99.09% <100.00%> (+0.02%)	⬆️
controller_manager/controller_manager/spawner.py	70.96% <50.00%> (-0.70%)	⬇️

... and 1 file with indirect coverage changes

[christophfroehlich]

LGTM

[bmagyar]

Thank you!