Change the default /boot part on s390x to not be lvm. (#1147501)

Avoid any insidious problems that may result from having /boot on lvm
on s390x, and simply change the default partitioning for /boot to be
on a partition.

Resolves: rhbz#1147501
Remove all traces of singlePV. (#1147501)

Won't be needing this anymore.

Related: rhbz#1147501