[cluster priority] change how it is defined

If no priority has been defined in cluster.yaml, the priority is now added to the generated json files

• based on manufactured_at instead of created_at
• with +100 for cluster with gpu_devices The generator oar_properties now only reads the value from the json file.

data/grid5000/sites/grenoble/clusters/drac/drac.json

@@ -919,6 +919,7 @@
     }
   ],
   "model": "IBM PowerNV S822LC (8335-GTB)",
+  "priority": 201710,
   "queues": [
     "default",
     "admin"