Lazily load resources when loading a Stack

Stack-list is one of heat's slowest operations.
It uses the orm to fetch resources for every stack
in the database, then spends a lot of time deserializing
resource data. This patch forces resources and dependencies
do be lazily loaded.

bug 1214602
Change-Id: Ieec481cb8ab211054f8956090ad97a687c9a616b
Merge "Lazily load resources when loading a Stack"