Ignition does not support nested RAID

Issue by @ajeddeloh

Issue Report

Bug

Container Linux Version

$ cat /etc/os-release
NAME="Container Linux by CoreOS"
ID=coreos
VERSION=1506.0.0+2017-08-28-1705
VERSION_ID=1506.0.0
BUILD_ID=2017-08-28-1705
PRETTY_NAME="Container Linux by CoreOS 1506.0.0+2017-08-28-1705 (Ladybug)"
ANSI_COLOR="38;5;75"
HOME_URL="https://coreos.com/"
BUG_REPORT_URL="https://issues.coreos.com"
COREOS_BOARD="amd64-usr"

Environment

Any

Expected Behavior

Ignition can create nested raids

Actual Behavior

Ignition fails to create nested raids.

Reproduction Steps

Use this CLC to create an instance.

storage:
  disks:
    - device: /dev/vdb
      wipe_table: true
      partitions:
       - label: root11
         number: 1
         size: 256MiB
         start: 0
         type_guid: be9067b9-ea49-4f15-b4f6-f36f8c9e1818
       - label: root21
         number: 2
         size: 256MiB
         start: 0
         type_guid: be9067b9-ea49-4f15-b4f6-f36f8c9e1818
       - label: root12
         number: 3
         size: 256MiB
         start: 0
         type_guid: be9067b9-ea49-4f15-b4f6-f36f8c9e1818
       - label: root22
         number: 4
         size: 256MiB
         start: 0
         type_guid: be9067b9-ea49-4f15-b4f6-f36f8c9e1818
  raid:
    - name: "inner1"
      level: "raid1"
      devices:
        - "/dev/vdb1"
        - "/dev/vdb2"
    - name: "inner2"
      level: "raid1"
      devices:
        - "/dev/vdb3"
        - "/dev/vdb4"
    - name: "outer"
      level: "raid0"
      devices:
        - "/dev/md/inner1"
        - "/dev/md/inner2"
  filesystems:
    - name: "ROOT"
      mount:
        device: "/dev/md/outer"
        format: "ext4"
        create:
          options:
            - "-L"
            - "ROOT"
    - name: "NOT_ROOT"
      mount:
        device: "/dev/vda9"
        format: "ext4"
        create:
          options:
            - "-L"
            - "WASTELAND"

Other Information

createRaids creates a list of devices it needs to make the arrays and waits for those devices before it creates the arrays. In the case of nested raid this includes devices it has yet to create.

Start at createRaids and trace how RAID devices are collected and waited on. Reproduce the supplied CLC; done means the inner arrays are created before the outer array, allowing the filesystem configuration to proceed.