Make baremetal_deploy_helper understand ephemeral disks

Teach baremetal_deploy_helper how to run sfdisk with ephemeral disks.
baremetal_deploy_helper calls default mkfs on the epheral disk.

Change-Id: I92d473c34e9a076f5dbcd54be538410b30b10204
Partial-Bug: #1238311
Co-Authored-By: Robert Collins <rbtcollins@hp.com>
Merge "Make baremetal_deploy_helper understand ephemeral disks"