Using 'istioConfig' throws invalid JSON payload error

Hi,

I have been trying to use 'istioConfig' to enable Istio on a GKE cluster.
I am using python templates in combination with[ hierarchical configuration setup for organization and department](https://github.com/GoogleCloudPlatform/deploymentmanager-samples/tree/master/community/hierarchical_configuration/Organization_with_departments) provided in the deployment manager samples.

The config for GKE cluster looks as follows in the python config file for the module:

```
config = {
"properties": {
"clusterLocationType": "Zonal",
"zone": "us-east1-b",
"cluster": {
"initialNodeCount": 6,
"nodeConfig": {
"machineType": "g1-small"
},
"addonsConfig": {
"httpLoadBalancing": {},
"horizontalPodAutoscaling": {},
"kubernetesDashboard": {
"disabled": True
},
"istioConfig": {}
}
}
}
}
```

Upon running deployment manager, I get following:

```
message: "{\"ResourceType\":\"container.v1.cluster\",\"ResourceErrorCode\":\"400\"\
,\"ResourceErrorMessage\":{\"code\":400,\"message\":\"Invalid JSON payload received.\
\ Unknown name \\\"istioConfig\\\" at 'cluster.addons_config': Cannot find field.\"\
,\"status\":\"INVALID_ARGUMENT\",\"details\":[{\"@type\":\"type.googleapis.com/google.rpc.BadRequest\"\
,\"fieldViolations\":[{\"field\":\"cluster.addons_config\",\"description\":\"\
Invalid JSON payload received. Unknown name \\\"istioConfig\\\" at 'cluster.addons_config':\
\ Cannot find field.\"}]}],\"statusMessage\":\"Bad Request\",\"requestPath\":\"\
https://container.googleapis.com/v1/projects/testproject/zones/us-east1-b/clusters\"\
,\"httpMethod\":\"POST\"}}"
```

I am not sure if this is a bug or if I have misconfigured the istio config.

Regards,
Giriraj