ceph-volume: fix bug with miscalculation of required db/wal slot size for VGs with multiple PVs
Previous logic for calculating db/wal slot sizes made the assumption that there would only be
a single PV backing each db/wal VG. This wasn't the case for OSDs deployed prior to v15.2.8,
since ceph-volume previously deployed multiple SSDs in the same VG. This fix removes the
assumption and does the correct calculation in either case.