or more generally, do not switch from current scheme to default one if
not told otherwise

else we get errors when partitionning after clearing them, such as:
"INTERNAL ERROR: unknown device vda7"

same rationale as in commit 767048570e8c44061cb0d6faf689698d3313870c for
mga#18666

this wasn't an issue as we later round partition on 1MB boundary but
it's still cleaner/safer...

rationale: write_*() takes (bootloader, all_hd, ext) as parameters (when
called from installkernel -> bootloader-config and then only when
migrating to uuids

as we don't care for 2 first for grub2, just ignore the 3rd too