Add preserve_ephemeral option to rebuild

The preserve_ephemeral option for rebuild will preserve the content of
the ephemeral partition, making stateful golden image based
deployments possible even when clouds haven't deployed Cinder, or the
hypervisor doesn't support Cinder (e.g. BareMetal / Ironic).

Partial-Bug: #1174154
blueprint: baremetal-preserve-ephemeral
Co-Authored-By: Robert Collins <rbtcollins@hp.com>

Change-Id: Id33d5d4107f89814842a3f0b7f33690dd7e3aadc
Merge "Add preserve_ephemeral option to rebuild"