[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/Setups section in the CS forces the system section 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 Setps are dropped from the CS

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

ENDRELEASENOTES