To install Ubuntu 18.04 on nodes using preseed Ubunutu 18.04, install on the head node Ubuntu 18.04 with Salt Master.
The head node-salt master will have PXE boot installed (via salt). The procedure is here:
https://cs.uwaterloo.ca/cscf/internal/edocs/Documentation/SALT/Pxe-boot-Ub1804.pdf
and the salt states and pillars files are here as tar file: https://cs.uwaterloo.ca/cscf/internal/edocs/Documentation/SALT/