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>

heat/tests/test_autoscaling.py

@@ -121,7 +121,7 @@ class AutoScalingTest(unittest.TestCase):
         self.assertEqual('WebServerGroup', resource.FnGetRefId())
         self.assertEqual('WebServerGroup-0', resource.resource_id)
         self.assertEqual(asc.AutoScalingGroup.UPDATE_REPLACE,
-                         resource.handle_update())
+                         resource.handle_update({}))
 
         resource.delete()
         self.m.VerifyAll()