Thanks.
In case some find this post useful, here's the codes.
api_params = {"Instances.Ec2SubnetId":subnet_id}
#Launching the cluster
cluster_id = conn.run_jobflow(
name,
ec2_keyname=AWS_EC2_KEY_NAME,
steps=[],
keep_alive=False,
master_instance_type = instance_type,
slave_instance_type = instance_type,
num_instances=2,
ami_version=ami_version,
api_params=api_params)