Installs the grub bootloader to the PreP Boot partition when the
prep_boot_partition_uuid is provided. This is required when
booting a partition image locally on ppc64* systems.
This change also passes the cpu_arch along to work_on_disk so
that the PReP partition is created when partitioning disks for
local boot on ppc64* systems,
Change-Id: I70667d43af962b357e6eeccba258f4fa5a91a09e
Depends-On: I2bc9f13ec605de7b7b96d96a1a4edebee0af76dc
Story: #1749057
Task: #22999