Always resolve properties against the current stack

During an update, resources can get moved around between stacks. Ensure
that the properties are always resolved against whatever the current stack
is, without needing to replace self.properties.

Change-Id: I09f984cdf4478ca7dda2b0a0f1aa5a63dd2c19bc
Add a "rollback" parameter to StackUpdate

We can only consider restoring backup resources during a rollback
operation. If they are present during a standard update, there is no way of
testing whether or not they match the new template definition, so we need
to treat them differently in this case.

Change-Id: Ibd73ed047486c253a6c06b7340a3a591f0fee3db
Merge "Always resolve properties against the current stack"
Merge "Add a "rollback" parameter to StackUpdate"