Use any disk for /boot regardless of its size

[OpenStack Infra]

https://review.openstack.org/327549
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.

commit 5cc463edfbc04e763cb77c01c3e5a3b430b778be
Author: Alexander Gordeev <email address hidden>
Date: Thu Jun 2 15:53:57 2016 +0300

    Use any disk for /boot regardless of its size

    Apparently, the disks which're bigger than 2T were excluded from list
    of bootable disks if possible. Just because in the past, those disks
    were unrecognized by BIOS (or UEFI under CSM).

    In fact, it was just misconfiguration of RAID controller or BIOS
    itself.

    GRUB uses BIOS INT13h in order to find all available disks.
    Therefore, unless particular disk is not configured as 'bootable',
    there's no change for GRUB to find it.

    One should configure hardware in the following way assuming that
    the first disk (hd0) is bootable and is used for operating system
    purposes.
    In case of hardware RAID, FC multipath or any other HBA, the disk
    (or lun, or whatever) which was configured as 'bootable' will be
    reported as hd0 via INT13h. So, GRUB will be able to boot from it.

    DocImpact
    Closes-Bug: #1588260

    Change-Id: I7bc729ffafa3b9d6bfe8521fa38599d36d02f7e1
    (backported from commit 8c118a477603e3c75e336c3a5cc9b36cc9edc05f)