alicloud：Got exception running Create: No JSON object could be decoded

run： ./pkb.py --cloud=AliCloud --machine_type=ecs.s2.large --benchmarks=iperf
get error about：
2016-06-04 00:56:14,230 6b9ee154 Thread-1 iperf(1/1) ERROR Got exception running Create: No JSON object could be decoded
2016-06-04 00:56:31,563 6b9ee154 Thread-1 iperf(1/1) INFO Waits until the status of VPC is in status_list: ['Available']
2016-06-04 00:56:31,566 6b9ee154 Thread-1 iperf(1/1) INFO Running: aliyuncli ecs DescribeVpcs --RegionId us-west-1 --VpcId vpc-u1dr5iduo
2016-06-04 00:56:32,458 6b9ee154 Thread-1 iperf(1/1) INFO Running: aliyuncli ecs CreateVSwitch --VSwitchName perfkit-6b9ee154-47c2823ab87c --ZoneId us-west-1a --CidrBlock 10.0.0.0/24 --VpcId vpc-u1dr5iduo