Show bm deploy how to preserve ephemeral content.

This changes the baremetal deploy helper to be able to preserve the
ephemeral partition, if the deploy requests that. The next patch in
the series makes it possible to store that intent in the deploy and
connects it to the deploy() method.

Partial-Bug: #1174154
blueprint: baremetal-preserve-ephemeral
Change-Id: I6bf01e52589c5894eb043f2b57e915d52e81ebc3
Merge "Show bm deploy how to preserve ephemeral content."