Automatically add biosboot partition on >2TB drives with RHEL 7/8

[kiryanov]

The PR is to fix issue

An error message "Your BIOS-based system needs a special partition to boot from a GPT disk label. To continue, please create a 1MiB 'biosboot' type partition. " is displayed on rinstall console and the Anaconda installation fails on >2TB drives, for which disklabel is automatically switched from msdos to gpt.

The modification include

Two pre-install scripts: share/xcat/install/scripts/pre.rh.rhels7 share/xcat/install/scripts/pre.rhels8

The UT result

##The UT output##

The content of the PR:

A similar 3-line code snippet is added to both scripts. It checks the $instdisk size using `blockdev --getsz' command and if it exceeds 2TB a one-megabyte biosboot partition is added to the partitionfile. Reference: https://bugzilla.redhat.com/show_bug.cgi?id=1032428

The UT result

##The UT output##