* ipxe_enabled was removed in Ussuri, now there is a separate ipxe boot
interface.
* iPXE now has its own set of configuration for the bootfile and config
template, and the values previously set when iPXE is enabled are now
the default in ironic. The overrides have been removed, since they
match the iPXE defaults.
Change-Id: I9d9f030ee4be979d0a849b59e5eb991f2d82f6a4