Ironic has changed the default PXE to be iPXE (as opposed to plain
PXE) in Yoga. Kolla Ansible supports either one or the other and
we tend to stick to upstream defaults so this change enables
iPXE instead of plain PXE - by default - the users are allowed
to change back and they need to take one other action so it is
good to remind them via upgrade notes either way.

Change-Id: If14ec83670d2212906c6e22c7013c475f3c4748a