Pass a Stack entity to Stack.load().

Pass a Stack entity to Stack.load().

Every call to Stack.load (except one) is loading models.Stack then passing just
the ID to Stack.load, which then loads models.Stack again.

As far as I can tell, SQLAlchemy doesn't have a 2nd level cache, so this results
in the db being queried twice.

This change makes it possibe to pass a stack *or* an ID to Stack.load, and
converts all calls to pass in a stack if it is available.

Change-Id: I501ebd403a241cc3b4a5c1e3070137cfc360bbda