We don't have a really working way to detect root UUID for whole
disk images at the moment, which results in an ignored traceback
every time install_bootloader is called with whole disk images in
UEFI mode. Avoid it by skipping GRUB2 if root UUID is unknown.
Change-Id: I84245538f59c664b72d1cafbca8d61be0978f489
(cherry picked from commit fc4e0eed6a)