When enable_ironic_ipxe is set in /etc/kolla/globals.yml,
the following happens:

- a new docker container, ironic_ipxe, is created. This contains
  an apache webserver used to serve up the boot images
- ironic is configured to use ipxe

Change-Id: I08fca1864a00afb768494406c49e968920c83ae7
Implements: blueprint ironic-ipxe