Use any disk for /boot regardless of its size

https://review.openstack.org/331057
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commit 1191ffe8dd69759fa2e672c44f53bdefc591027d
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