'Starts a new task from the specified task definition on the
specified container instance or instances. If you want to use
the default Amazon ECS scheduler to place your task, use
`RunTask` instead.
:type cluster: string
:param cluster: The short name or full Amazon Resource Name (ARN) of
the cluster that you want to start your task on. If you do not
specify a cluster, the default cluster is assumed..
:type task_definition: string
:param task_definition: The `family` and `revision` (
`family:revision`) or full Amazon Resource Name (ARN) of the task
definition that you want to start.
:type overrides: dict
:param overrides:
:type container_instances: list
:param container_instances: The container instance UUIDs or full Amazon
Resource Name (ARN) entries for the container instances on which
you would like to place your task.'

def start_task(self, task_definition, container_instances, cluster=None, overrides=None):

    params = {'taskDefinition': task_definition}
    self.build_list_params(params, container_instances, 'containerInstances.member')
    if (cluster is not None):
        params['cluster'] = cluster
    if (overrides is not None):
        params['overrides'] = overrides
    return self._make_request(action='StartTask', verb='POST', path='/', params=params)
