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
Merge "Make the PXE driver understand ephemeral disks"