[ros2_control_node] Handle simulation environment clocks

[saikishor]

As there are many ongoing issues that people are trying to use the ros2_control_node for the simulation environments and as it is not working, I wanted to see if this solution is something that we are interesting to go with or we simple reject the possibility

Related to : #859

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.61%. Comparing base (d55def1) to head (bb5f0f1).

Files with missing lines	Patch %	Lines
controller_manager/src/ros2_control_node.cpp	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1810      +/-   ##
==========================================
- Coverage   87.62%   87.61%   -0.01%     
==========================================
  Files         120      120              
  Lines       12217    12226       +9     
  Branches     1093     1094       +1     
==========================================
+ Hits        10705    10712       +7     
- Misses       1123     1124       +1     
- Partials      389      390       +1     

Flag	Coverage Δ
unittests	87.61% <66.66%> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
controller_manager/src/ros2_control_node.cpp	84.21% <66.66%> (-4.03%)	⬇️

... and 5 files with indirect coverage changes

[fmauch]

In which situations would this be useful? When we have a simulator hardware component that doesn't run its own control node such as Gazebo, right?

@saikishor which issues are related to this PR?

[saikishor]

In which situations would this be useful? When we have a simulator hardware component that doesn't run its own control node such as Gazebo, right?

@saikishor which issues are related to this PR?

@fmauch It seems many are using this node alongside IssacSim or other simulators. We should decide what to do in our next WG meeting, either support it this way or throw an exception that it is not supported to use with simulation.

It is related to #859 and ros-controls/ros2_controllers#537