[8.0] Drop the system instance level in the CS

[atsareg]

This PR allows to drop the System instance section level from the CS. The presence of the /DIRAC/NoSetup=True option in the CS suggests that the configuration is defined without Setups (to be introduced in 9.0 #7671). This forces the system section not to contain the instance level in the output of getSystemSection() and similar cases. This way it should be compatible with CS both with and without Setups defined.

BEGINRELEASENOTES

*Configuration
CHANGE: PathFinder.getSystemInstance() returns empty string if Setups are dropped from the CS

*WorkloadManagement
CHANGE: StalledJobAgent, Watchdog - use getSystemSection() instead of getSystemInstance()

ENDRELEASENOTES

[fstagni]

So, IIUC:

• this PR can be merged, deployed, and nothing should happen
• at some point in time, the admins should:
  • set the DIRAC/NoSetup=True AND
  • remove the Systems part of CS to remove the instance parts
  • the /DIRAC/Setups part of CS can be entirely removed
• at this point update to v9 can happen "freely"

At a minimum we need some instructions, to be added to the v9 wiki. A script that does the CS update would be better.