Ironic: Support both plain PXE and iPXE

ansible/roles/ironic/defaults/main.yml

ansible/roles/ironic/handlers/main.yml

ansible/roles/ironic/tasks/bootstrap_service.yml

ansible/roles/ironic/tasks/check-containers.yml

ansible/roles/ironic/tasks/upgrade.yml

ansible/roles/ironic/templates/ironic-conductor.json.j2

ansible/roles/ironic/templates/ironic-dnsmasq.conf.j2

ansible/roles/ironic/templates/ironic-ipxe-httpd.conf.j2

ansible/roles/ironic/templates/ironic-ipxe.json.j2

ansible/roles/ironic/templates/ironic-pxe.json.j2

ansible/roles/ironic/templates/ironic.conf.j2

ansible/roles/ironic/templates/ironic_pxe_uefi.default.j2

doc/source/reference/bare-metal/ironic-guide.rst

etc/kolla/globals.yml

releasenotes/notes/ironic-default-ipxe-1e9bb7a3f4994d84.yaml

releasenotes/notes/ironic-pxe-and-ipxe-57357e928ed9ea48.yaml