curtin does not properly support MD raid device renaming (uses /dev/<name> instead of /dev/md/<name> when creating a device)

[Dmitrii Shcherbakov]

maas root machine get-curtin-config <system_id>
https://pastebin.canonical.com/p/DRwJYVbYjr/

Trying to set up a raid device with a custom name (os-raid):
      -
        devices:
          - os0-os-data
          - os1-os-data
        id: os-raid
        name: os-raid
        raidlevel: '1'
        spare_devices: []
        type: raid
      -
        fstype: ext4
        id: os-raid-format
        label: 'os'
        type: format
        volume: os-raid
      -
        device: os-raid-format
        id: os-raid-mount
        path: /
        type: mount

Curtin fails to do that while invoking mdadm as it uses /dev/os-raid instead of /dev/md/os-raid:

https://github.com/CanonicalLtd/curtin/blob/ubuntu/18.2-10-g7afd77fa-0ubuntu1_18.10.1/curtin/commands/block_meta.py#L1036
    md_devname = block.dev_path(info.get('name'))
# ...
    mdadm.mdadm_create(md_devname, raidlevel,
                       device_paths, spare_device_paths,
                       info.get('mdname', ''))

https://github.com/CanonicalLtd/curtin/blob/ubuntu/18.2-10-g7afd77fa-0ubuntu1_18.10.1/curtin/block/__init__.py#L55-L62
def dev_path(devname):
    """
    convert device name to path in /dev
    """
    if devname.startswith('/dev/'):
        return devname
    else:
        return '/dev/' + devname

curtin: Installation started. (18.1)
third party drivers not installed or necessary.
An error occured handling 'os-raid': ProcessExecutionError - Unexpected error while running command.
Command: ['mdadm', '--create', '/dev/os-raid', '--run', '--homehost=block-storage-6', '--level=1', '--raid-devices=2', '/dev/sdf3', '/dev/sdh1']
Exit code: 1
Reason: -
Stdout: ''
Stderr: mdadm: Note: this array has metadata at the start and
            may not be suitable as a boot device. If you plan to
            store '/boot' on this device please ensure that
            your boot-loader understands md/v1.x metadata, or use
            --metadata=0.90
        mdadm: /dev/os-raid is an invalid name for an md device. Try /dev/md/os-raid

Unexpected error while running command.
Command: ['mdadm', '--create', '/dev/os-raid', '--run', '--homehost=block-storage-6', '--level=1', '--raid-devices=2', '/dev/sdf3', '/dev/sdh1']
Exit code: 1
Reason: -
Stdout: ''
Stderr: mdadm: Note: this array has metadata at the start and
            may not be suitable as a boot device. If you plan to
            store '/boot' on this device please ensure that
            your boot-loader understands md/v1.x metadata, or use
            --metadata=0.90
        mdadm: /dev/os-raid is an invalid name for an md device. Try /dev/md/os-raid

curtin: Installation failed with exception: Unexpected error while running command.
Command: ['curtin', 'block-meta', 'custom']
Exit code: 3
Reason: -
Stdout: An error occured handling 'os-raid': ProcessExecutionError - Unexpected error while running command.
        Command: ['mdadm', '--create', '/dev/os-raid', '--run', '--homehost=block-storage-6', '--level=1', '--raid-devices=2', '/dev/sdf3', '/dev/sdh1']
        Exit code: 1
        Reason: -
        Stdout: ''
        Stderr: mdadm: Note: this array has metadata at the start and
                    may not be suitable as a boot device. If you plan to
                    store '/boot' on this device please ensure that
                    your boot-loader understands md/v1.x metadata, or use
                    --metadata=0.90
                mdadm: /dev/os-raid is an invalid name for an md device. Try /dev/md/os-raid

        Unexpected error while running command.
        Command: ['mdadm', '--create', '/dev/os-raid', '--run', '--homehost=block-storage-6', '--level=1', '--raid-devices=2', '/dev/sdf3', '/dev/sdh1']
        Exit code: 1
        Reason: -
        Stdout: ''
        Stderr: mdadm: Note: this array has metadata at the start and
                    may not be suitable as a boot device. If you plan to
                    store '/boot' on this device please ensure that
                    your boot-loader understands md/v1.x metadata, or use
                    --metadata=0.90
                mdadm: /dev/os-raid is an invalid name for an md device. Try /dev/md/os-raid

Stderr: ''