Add support for rebuild --preserve-ephemeral.

This new extension going into Nova permits preserving the ephemeral
partition during a rebuild, getting enough functionality to preserve
state - e.g. on baremetal instances - without Cinder.

DocImpact
Change-Id: Ibc4c24bb2efb82c11699231ae76e2755a753a706
Merge "Add support for rebuild --preserve-ephemeral."