Fixes for raid0-lvm partman config

Only create the grub partion on GPT disks and set it to 10MB size and
not 1 as the standard says at least one. 10 was chosen arbitrarily.
Declare lvmignore for all raid partitions
Declare anyway boot_degraded
Instruct lvm to remove pre-existing LVs

Change-Id: Ib1051cec27943b7bdddfb9b0e4a1f9cfdffbf31d