Fix incorrect logic during updating in unittest

The property image_update_policy has two ways for initialization during
update. One of them is when image_update_policy is not presented in
prop_diff and shoud be used from self.properties. So image_update_policy
should be used in old and new templates without any changes.

Related-Bug: #1298850
Change-Id: Ie9afa65ad6d095d4293be077536fb807919de789
Merge "Fix incorrect logic during updating in unittest"