'List EC2 instance groups in a cluster
:type cluster_id: str
:param cluster_id: The cluster id of interest
:type marker: str
:param marker: Pagination marker'

def list_instance_groups(self, cluster_id, marker=None):

    params = {'ClusterId': cluster_id}
    if marker:
        params['Marker'] = marker
    return self.get_object('ListInstanceGroups', params, InstanceGroupList)
