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

---

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

[mergify]

Cherry-pick of 9e57adf has failed:

On branch mergify/bp/iron/pr-1717
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Changes to be committed:
	modified:   controller_manager/controller_manager/spawner.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   controller_manager/test/test_spawner_unspawner.cpp

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 9 lines in your changes missing coverage. Please review.

Project coverage is 62.39%. Comparing base (0465212) to head (137dae7).
Report is 1 commits behind head on iron.

Files with missing lines	Patch %	Lines
controller_manager/controller_manager/spawner.py	50.00%	3 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             @@
##             iron    #1843      +/-   ##
==========================================
- Coverage   62.41%   62.39%   -0.03%     
==========================================
  Files         112      112              
  Lines       12969    12984      +15     
  Branches     8766     8782      +16     
==========================================
+ Hits         8095     8101       +6     
- Misses        947      952       +5     
- Partials     3927     3931       +4     

Flag	Coverage Δ
unittests	62.39% <57.14%> (-0.03%)	⬇️

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

Files with missing lines	Coverage Δ
controller_manager/test/test_spawner_unspawner.cpp	57.57% <63.63%> (+0.18%)	⬆️
controller_manager/controller_manager/spawner.py	70.40% <50.00%> (-0.68%)	⬇️

... and 2 files with indirect coverage changes