Do updates based on a single dependency list

This ensures that updates proceed in the correct order with maximum
parallelism, for arbitrarily complex dependency graphs.

Change-Id: Ia11f4cfef58a3045199e1e5e49050cb1646f5057
Don't stub LoadBalancer.update in autoscaling tests

Stub handle_update() instead. We can still verify that the arguments are as
expected, while still exercising as much of the logic as possible.

Change-Id: Iebb861bb5af27d92e65083c8bac6821f91382dd5
Merge "Do updates based on a single dependency list"
Merge "Don't stub LoadBalancer.update in autoscaling tests"