Fix unload of controllers when spawned with --unload-on-kill (backport #1717)

[mergify]

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

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This is an automatic backport of pull request #1717 done by Mergify.

[mergify]

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Unmerged paths:
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	both modified:   controller_manager/controller_manager/spawner.py
	both modified:   controller_manager/test/test_spawner_unspawner.cpp

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[codecov]

Codecov Report

Attention: Patch coverage is 54.54545% with 10 lines in your changes missing coverage. Please review.

Project coverage is 62.57%. Comparing base (d897740) to head (9bfa9b7).
Report is 2 commits behind head on humble.

Files with missing lines	Patch %	Lines
controller_manager/controller_manager/spawner.py	45.45%	4 Missing and 2 partials ⚠️
controller_manager/test/test_spawner_unspawner.cpp	63.63%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           humble    #1842      +/-   ##
==========================================
- Coverage   62.58%   62.57%   -0.01%     
==========================================
  Files         108      108              
  Lines       12026    12041      +15     
  Branches     8133     8145      +12     
==========================================
+ Hits         7526     7535       +9     
- Misses        880      882       +2     
- Partials     3620     3624       +4     

Flag	Coverage Δ
unittests	62.57% <54.54%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
controller_manager/test/test_spawner_unspawner.cpp	58.89% <63.63%> (+0.15%)	⬆️
controller_manager/controller_manager/spawner.py	68.46% <45.45%> (-0.59%)	⬇️