Populating flavor details from finish_resize.start

By the time Nova starts the finish_resize phase, the flavor has officially changed. Thus, it's safe to use the flavor details of the finish_resize.start notification. The reason for doing this is so we're aware of the flavor change if the instance is deleted mid-finish_resize. If that happens, the final exists for the instance is emitted with the new flavor details and before the correct exists would fail verification. Now, that exists will pass verification like it should.