heat engine : don't replace resource template before update

We want to validate the pre-update properties without touching
the existing template, because the resource implementation will
need to compare old/new json snippets to decide if a and update
can be performed or not.

ref blueprint instance-update-stack

Change-Id: I37ce6e799242819de6104a53f8458d8bbc3abf1c
Signed-off-by: Steven Hardy <shardy@redhat.com>
Merge "heat engine : don't replace resource template before update"