slap/standalone: fix format with 0 partitions
we can have a case where written slapos.cfg has "partition_amount = 10" (leftover from previous execution) and we call format(0), in this case, we really want slapos.cfg function to be rewritten.
we can have a case where written slapos.cfg has "partition_amount = 10" (leftover from previous execution) and we call format(0), in this case, we really want slapos.cfg function to be rewritten.