[2.1] maas admin discoveries scan API output shows rack controller ids instead of names

[Andres Rodriguez]

roaksoax@unleashed:~/Desktop/project/maas> maas admin discoveries scan
Success.
Machine-readable output follows:
{
    "result": "Scanning is in-progress on all rack controllers.",
    "scan_attempted_on": [
        "panpsd"
    ],
    "scan_started_on": [
        "panpsd"
    ],
    "failed_to_connect_to": [],
    "scan_already_in_progress_on": [],
    "rpc_call_timed_out_on": []
}

The output is really good, but we need to improve on:

 - Instead of rack controller ids, it should show the rack controller names. But for better results rack name and id.
 - It doesn't tell you in what subnets the scan will be done. It should.