Fix rollback failure when updating stack

When building oldstack in rollback, self.env doesn't match self.t, then
the validating will fail, but oldstack is useless in rollback, so we don't
need to build it in rollback.

Change-Id: Iab17824d108d565126c1dd0338fefc9a5ee4f47b
Closes-Bug: #1308883
Merge "Fix rollback failure when updating stack"