heat engine : add Resource update_template_diff method

heat engine : add Resource update_template_diff method

Add method to top-level Resource class update_template_diff()
which allows us to determine the difference between old/new
templates for a resource update, and also raise an error
if the changed keys are not in the list of those defined
as supported for update by the resource

ref blueprint instance-update-stack

Change-Id: Ibef07a0cecbc15f7f1d6c2c663743e3af8023057
Signed-off-by: Steven Hardy <shardy@redhat.com>