Implement the TemplateOptions.networks option in Azure ARM

The Azure ARM compute service does not implement the TemplateOptions.networks option. It creates a default network with a range taken from the context properties and a subnet in it, and all created nodes are assigned there.

Having a default network and subnet is fine when users don't provide networking information, but we should support the TemplateOptions.networks option to connect the VMs to the subnets with the given IDs. We could also support more sophisticated networking options in the AzureTemplateOptions class to let users configure not only the subnet but also the ip configuration (static, dynamic), as allowed by the provider.