In a new installation, the first server being built will fail when reaching the disk partition section and complain on a missing diskpart.cfg

The solution is to add the following two lines to the partition tables:
cat <<EOF > /tmp/diskpart.cfg
EOF

such that the final partition template looks like:

#Dynamic
cat <<EOF > /tmp/diskpart.cfg
zerombr
clearpart --all --initlabel
part /boot --fstype ext3 --size=500 --ondisk=sda
part swap --size=1024 --ondisk=sda
part pv.01 --size=1024 --grow --ondisk=sda
volgroup vg_root pv.01
logvol / --vgname=vg_root --size=1 --grow --name=lv_root
EOF