Cells: Pass instance objects to build_instances

When the cells scheduler creates instance records in its local db it
should keep the instance objects created.  build_instances expects a
list of instance objects so to keep parity between cells and non-cells
the cells scheduler needs to call build_instances with its list of
instance objects.

Part of bp remove-cast-to-schedule-run-instance

Change-Id: I4a1a96693f9c3cb92ffdccfebaeb2432584d4105
Merge "Cells: Pass instance objects to build_instances"