ipxe-bootimgs will soon include the ipxe efi variant
ipxe-snponly.efi which has no networking stack, relying instead
on the UEFI networking stack. This image is required in some UEFI
environments instead of the full ipxe.efi, so there needs to be
a mechanism for serving it by default.
This change makes this possible by setting the puppet parameter
ironic::pxe::ipxe_name_base to ipxe-snponly.
(cherry picked from commit