storage: add option to not persist units on store_instance
runners/instance_scalator: call store_instance with save_units=False

When we want to update just the state of the instance, we don't need to
save all units. This may lead to bad behavior.
runners/instance_starter: call store_instance with save_units=False

When we want to update just the state of the instance, we don't need to
save all units. This may lead to bad behavior.
feaas: bump to 0.7.2