Asymmetrical treatment of "priority" and "policy"

[douglas-raillard-arm]

"priority" and "policy" keys are not treated on the same ground.

Given the base configuration:

{
	  "global": {
		    "duration": -1,
		    "calibration": 265,
		    "lock_pages": false,
		    "logdir": "./"
	  },
	  "tasks": {
		    "invar0": {
		        "loop": 1,
		        "phases": {
			          "phase1": {
			              "policy": "SCHED_FIFO",
			              "priority": 15,
			              "run": 10,
			          },
			          "phase2": {
			              "run": 10,
			          }
		        }
		    }
	  }
}

If "phase2" gets policy=SCHED_FIFO, the priority will be reset to the default value instead of 15 being preserved. If "phase2" gets priority=15, the policy will not get reset and will still be FIFO.