Proper examples for a deployment with network_data

[NobbZ]

After #54 beeing an obvious fail we dug further in the code and found that there is already some support in the deployment ressource for network_data.

The example in the README just uses network_data = var.network_data (https://github.com/openshift-metal3/terraform-provider-ironic#deployment)

Is there a more complete example about how to set the network_data up?

Our team tried several variants, but it always resulted in a typeerror, as nested values have to be strings (according to TF), while the structure of the network data usually is a JSON object and would roughly have the type map[string][]map[string]interface{} in Go parlance.