Support preserve_ephemeral in baremetal.

We need to be able to capture the semantic intent of a rebuild with
ephemeral preservation for the baremetal helper, which means another
column in bm.modes.

Partial-Bug: #1174154
blueprint: baremetal-preserve-ephemeral
Co-Authored-By: Roman Podoliaka <rpodolyaka@mirantis.com>
Change-Id: Icf627f38dac02b7d06ff0f1816741e5513a4fcf3
Merge "Support preserve_ephemeral in baremetal."