Add a new configuration to tests the training command

Would have been useful for this dev: !299 (merged) because tox doesn't test everything and we sometimes have to run the command ourselves.

configs/quickstart.json

@@ -71,7 +71,7 @@
             "ddp_port": "20027",
             "use_amp": true,
             "nb_gpu": null,
-            "force_cpu": false
+            "force": null
         },
         "metrics": {
             "train": [