Fix Hardware spawner and add tests for it

[fmauch]

This basically implements my comments from #1682 (review)

I discussed with @destogl that I would create a new PR including his commit from #1682. So, this effectively replaces #1682.

It basically changes the following things:

• Actually makes the hardware spawner work. I think it never worked correctly the way it was structured before. (is_hardware_component_loaded checks for one single string, while activate_components and configure_components expect a list.)
• Fully implement that the hardware spawner allows specifying a list of components that should be started. This is cmdline-interface-breaking (basically only on a parameter description level) change but since it never worked I would expect nobody has been using it so far, so we can safely backport it.
• Add tests for hardware_spawner (mainly implemented by @destogl)

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 87.46%. Comparing base (84e85f9) to head (ad8cfa5).

Files with missing lines	Patch %	Lines
...ler_manager/controller_manager/hardware_spawner.py	68.42%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1759      +/-   ##
==========================================
+ Coverage   86.79%   87.46%   +0.67%     
==========================================
  Files         116      117       +1     
  Lines       10715    10770      +55     
  Branches      981      980       -1     
==========================================
+ Hits         9300     9420     +120     
+ Misses       1062      991      -71     
- Partials      353      359       +6     

Flag	Coverage Δ
unittests	87.46% <92.00%> (+0.67%)	⬆️

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_hardware_spawner.cpp	100.00% <100.00%> (ø)
...ler_manager/controller_manager/hardware_spawner.py	67.85% <68.42%> (+67.85%)	⬆️

... and 1 file with indirect coverage changes