':param ConfigurationPersistenceService persistence_service: Service
for retrieving and setting desired configuration.
:param ClusterStateService cluster_state_service: Service that
knows about the current state of the cluster.
:param IReactorTime clock: The clock to use for time. By default
global reactor.'

def __init__(self, persistence_service, cluster_state_service, clock=reactor):

    self.persistence_service = persistence_service
    self.cluster_state_service = cluster_state_service
    self.clock = clock
