Partition tables are included literally but not processed by erb. Please make Partition tables erbish, the following should work.

part /boot --size=256
part pv.2 --size=0 --grow
volgroup vg_<%= @host.name -%> pv.2
logvol / --name=wurzel --vgname=vg_<%= @host.name -%> --size=3192
logvol /var --name=_var --vgname=vg_<%= @host.name -%> --size=1024 --
grow --maxsize=2048
logvol swap --fstype swap --name=swap --vgname=vg_<%= @host.name -%> --
size=2048