How to assign network ID for scenario boot-and-delete-server-with-keypairs.json

Scenario: boot-and-delete-server-with-keypairs.json
{
    "NovaKeypair.boot_and_delete_server_with_keypair": [
        {
            "args": {
                "flavor": {
                    "name": "default"
                },
                "image": {
                    "name": "cirros_0.3.3"
                },
            },
            "runner": {
                "type": "constant",
                "times": 5,
                "concurrency": 2
            },
            "context": {
                "users": {
                    "tenants": 2,
                    "users_per_tenant": 1
                },
                "network": {
                    "start_cidr": "100.1.0.0/26"
                }
            }
        }
    ]
}

Error Message:
2015-04-14 06:25:00.307 11472 INFO rally.benchmark.context.users [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | Starting: Enter context: `users`
2015-04-14 06:25:01.139 11472 INFO rally.benchmark.context.users [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | Completed: Enter context: `users`
2015-04-14 06:25:01.139 11472 INFO rally.benchmark.context.network [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | Starting: Enter context: `network`
2015-04-14 06:25:02.649 11472 INFO rally.benchmark.context.network [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | Completed: Enter context: `network`
2015-04-14 06:25:03.542 11699 INFO rally.benchmark.runners.base [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | ITER: 0 START
2015-04-14 06:25:03.542 11700 INFO rally.benchmark.runners.base [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | ITER: 1 START
2015-04-14 06:25:04.854 11699 INFO rally.benchmark.runners.base [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | ITER: 0 END: Error BadRequest: Multiple possible networks found, use a Network ID to be more specific. (HTTP 400) (Request-ID: req-efc1a262-c7be-481b-abc9-f504597cc780)
2015-04-14 06:25:04.854 11699 INFO rally.benchmark.runners.base [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | ITER: 2 START
2015-04-14 06:25:04.940 11700 INFO rally.benchmark.runners.base [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | ITER: 1 END: Error BadRequest: Multiple possible networks found, use a Network ID to be more specific. (HTTP 400) (Request-ID: req-cb920145-ce65-48aa-b598-7b4da80254e2)
2015-04-14 06:25:04.941 11700 INFO rally.benchmark.runners.base [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | ITER: 3 START
2015-04-14 06:25:05.893 11699 INFO rally.benchmark.runners.base [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | ITER: 2 END: Error BadRequest: Multiple possible networks found, use a Network ID to be more specific. (HTTP 400) (Request-ID: req-e509cef5-ced3-48d5-b0aa-2481cd060d20)
2015-04-14 06:25:05.893 11699 INFO rally.benchmark.runners.base [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | ITER: 4 START
2015-04-14 06:25:06.158 11700 INFO rally.benchmark.runners.base [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | ITER: 3 END: Error BadRequest: Multiple possible networks found, use a Network ID to be more specific. (HTTP 400) (Request-ID: req-ca88f241-9ef4-4242-b6c4-7bc84a53dc60)
2015-04-14 06:25:06.877 11699 INFO rally.benchmark.runners.base [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | ITER: 4 END: Error BadRequest: Multiple possible networks found, use a Network ID to be more specific. (HTTP 400) (Request-ID: req-0e898d76-45f1-4fec-b308-d19c904638ab)
2015-04-14 06:25:06.949 11472 INFO rally.benchmark.context.cleanup.context [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | Starting: user resources cleanup
2015-04-14 06:25:10.355 11472 INFO rally.benchmark.context.cleanup.context [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | Completed: user resources cleanup
2015-04-14 06:25:10.355 11472 INFO rally.benchmark.context.network [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | Starting: Exit context: `network`
2015-04-14 06:25:11.837 11472 INFO rally.benchmark.context.network [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | Completed: Exit context: `network`
2015-04-14 06:25:11.837 11472 INFO rally.benchmark.context.users [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | Starting: Exit context: `users`
2015-04-14 06:25:12.990 11472 INFO rally.benchmark.context.users [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | Completed: Exit context: `users`
2015-04-14 06:25:13.132 11472 INFO rally.benchmark.engine [-] Task 1e0e6d3c-872d-4444-8a27-0791cf0c357a | Completed: Benchmarking.

Analyze:
I checked the source code /rally/benchmark/scenarios/nova/keypairs.py:
    def boot_and_delete_server_with_keypair(self, image, flavor,
                                            **kwargs):
        """Boot and delete server with keypair.

        Plan of this scenario:
         - create a keypair
         - boot a VM with created keypair
         - delete server
         - delete keypair

        :param image: ID of the image to be used for server creation
        :param flavor: ID of the flavor to be used for server creation
        :param kwargs: Optional additional arguments for keypair creation
        """

        keypair = self._create_keypair(**kwargs)
        server = self._boot_server(image, flavor,
                                   key_name=keypair)
        self._delete_server(server)
        self._delete_keypair(keypair)

I found that when booting a server, it only pass three parameters (image, flavor, keypair) to self._boot_server(). How could I pass the network ID information to it?

Thanks.