heat engine : pass json snippet into resource handle_update

Change handle_update to accept the resource json_snippet,
needed to decide if a non-replacement update is possible
by comparing pre and post update resource definitions

ref blueprint instance-update-stack

Change-Id: I7eedce41eae232330f6fce4f098183cbd7fd6768
Signed-off-by: Steven Hardy <shardy@redhat.com>
Merge "heat engine : pass json snippet into resource handle_update"