Pod architectures are listed multiple times

Currently, we are listing the architectures found for each RSD API "System".

$ maas admin pods create type="rsd" power_address="10.3.0.1:8443" power_user="admin" power_pass="admin"
Success.
Machine-readable output follows:
{
    "architectures": [
        "amd64/generic",
        "amd64/generic",
        "amd64/generic",
        "amd64/generic",
        "amd64/generic",
        "amd64/generic",
        "amd64/generic",
        "amd64/generic",
    ],
    "name": "eager-yak",
    "resource_uri": "/MAAS/api/2.0/pods/135/",
    "total": {
        "local_disks": 7,
        "memory": 249984,
        "local_storage": 959999999999,
        "cores": 352
    },
    "capabilities": [
        "composable",
        "fixed_local_storage"
    ],
    "type": "rsd",
    "used": {
        "local_disks": 0,
        "memory": 0,
        "local_storage": 0,
        "cores": 0
    },
    "available": {
        "local_disks": 7,
        "memory": 249984,
        "local_storage": 959999999999,
        "cores": 352
    },
    "id": 135
}

We should only be storing one instance of each architecture found, in this case "amd64/generic", so that the user has an idea of what type of architectures the Pod supports.