Make the PXE driver understand ephemeral disks
Make the PXE driver understand ephemeral disks

Allows the PXE driver to create an ephemeral partition. This is the first
step in being able to redeploy a baremetal machine with a new image but
preserve the data on the ephemeral disk.

Based on Nova's patch: https://review.openstack.org/51735

blueprint: preserve-ephemeral
Closes-Bug: #1238311
Change-Id: I2826f1f8b60316ea180284cffc495a7556e31784

citrix-openstack-build/ironic-lib,openstack/ironic-lib,faizan-barmawer/ironic-lib,NaohiroTamura/ironic-lib,faizan-barmawer/elytics