Add enable_ironic_ipxe option to globals.yml

Kolla Ironic reference point out enable booting via iPXE by setting
"enable_ironic_ipxe" to "true" in "/etc/kolla/globals.yml", but there
is no enable_ironic_ipxe option exists in globals.yml.

Change-Id: I5794fc5a7d236f7f486011f53366a5b3f4c59731