Remove DHCP_DRIVER from the seed boot call.

The seed doesn't need variation at this point, and config.json is
painful to edit. Rather than adding more sed debt, I think we can
avoid parameterising the seed for boot-technology for now, and just
migrate it entirely once the neutron native PXE is stable.
Pure Havana can support Neutron native PXE - it just won't be able
to support no-file-injection etc.

Change-Id: I92d203c5e496ca5d688243c50dbc140f8cc238a4