We should provide option to setup PXE in order to remove PXE setup
from tripleo elements. Class ironic::pxe will setup tftpboot and
httpboot and class ironic::pxe::common will take care of common
dependancies between ironic, ironic inspector and pxe driver.
Change-Id: I8b83eff694316755e4dd2dbcde7b569472893bc5