Update: Keep a copy of the old template for rollback

To implement the blueprint update-failure-recovery we will need to modify
the current template as we change the stack. We'll need an unmodified copy
to roll back to in the event of failure, so base the previous stack on a
copy.

Change-Id: Ic06acd3c9a89312d1ad8922e26ab02bee4bafeb0
Merge "Update: Keep a copy of the old template for rollback"