Managing overflow sites like NERSC

Unified extends the whitelist to NERSC in real time when conditions are met which are the following:

• FNAL agent
• Idle jobs / Running jobs > 0.5
• Campaign is setup with overflow OR the workflow requires no primary input (like wmLHEGS)

 "overflow": {
      "PRIM": {}
    }

Module used in unified for managing overflow sites - equalizor.py