update_engine: Add support for enterprise rollback powerwash

- Adds the additional flag "rollback" to the powerwash file
- This flag allows additional data to be preserved over a powerwash
- Adds tests

BUG=chromium:881341
TEST=unittests
Change-Id: I4487f4de856ea8d2d0255e8de4cd1ba0762a8e53
Reviewed-on: https://chromium-review.googlesource.com/1412683


Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>