setup-disk: fix partitioning failure

Closes #10506 (closed). Previously, setup-disk could fail when installing over an existing identical partition table. The additions ensure that a clean disklabel is created and existing partitions are wiped.

Signed-off-by: Edin Taric unicorn@regrow.earth

Apologies for the duplicate MR, the MR workflow is quite confusing to me. I have made the readability change @ncopa requested and rebased.