[2.2] MAAS sets the last disk (i.e. sdf) as the boot device, instead of the first (i.e. sda)

When MAAS gathers the disk information, it outsputs a json that is later processed by MAAS. On a multiple disk settings:

1. MAAS discovers all disks, but the /last/ disk on the OS (e.g. sdf) is the /first/ one on the json. Example:

[
{
"ROTA": "1",
"NAME": "sdb",
"MODEL": "MM1000GFJTE",
"RM": "0",
"RO": "0",
"SIZE": "1000204886016",
"BLOCK_SIZE": "4096",
"ID_PATH": "/dev/disk/by-id/wwn-0x30014380408bad41",
"PATH": "/dev/sdb",
"SERIAL": "30014380408bad41"
},
{
"ROTA": "1",
"NAME": "sda",
"MODEL": "MM1000GFJTE",
"RM": "0",
"RO": "0",
"SIZE": "1000204886016",
"BLOCK_SIZE": "4096",
"ID_PATH": "/dev/disk/by-id/wwn-0x30014380408bad40",
"PATH": "/dev/sda",
"SERIAL": "30014380408bad40"
}
]

2. Once the disks are processed, this causes the /last/ disk the OS discovered, to become the /first/ disk added to MAAS.

3. Since the /first/ disk added to MAAS is the /last/ disk on the OS, the /last/ disk becomes the boot disk.

In most scenarios, the /last/ disk is never the /boot/ disk. This causes machine deployment failures provided that the root & boot are set on 'sdb', while machines are trying to boot from 'sda'.