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 93.68421% with 6 lines in your changes missing coverage. Please review.

Project coverage is 87.53%. Comparing base (0433960) to head (866c6e4).

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   86.83%   87.53%   +0.69%     
==========================================
  Files         121      122       +1     
  Lines       11542    11618      +76     
  Branches     1054     1054              
==========================================
+ Hits        10023    10170     +147     
+ Misses       1144     1070      -74     
- Partials      375      378       +3     

Flag	Coverage Δ
unittests	87.53% <93.68%> (+0.69%)	⬆️

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	70.23% <68.42%> (+70.23%)	⬆️

... and 2 files with indirect coverage changes

[destogl]

Thanks for fixing this! And yes, we have to backport this. Not many people are using it so it is fine if we do small breakings on user-level.

[saikishor]

The proposed changes good to me. One minor comment about the tests and another one to update the documentation

[saikishor]

LGTM

[christophfroehlich]

LGTM.

We should add the usage of this in the demos. Maybe it suits with ros-controls/ros2_control_demos#417.

And should we also add a load_hardware_component CLI verb?