Simplify providing additional_networks for Slurm nodesets

[wiktorn]

Simplify definition of additional_networks, before the change one had to write for example

           additional_networks:
             - network: ""
               subnetwork: $(network2.subnetwork_self_link)
               subnetwork_project: ""
               network_ip: ""
               nic_type: VIRTIO_NET
               stack_type: ""
               queue_count: null
               access_config: []
               ipv6_access_config: []
               alias_ip_range: []

Now its possible to write just:

          additional_networks:
            - subnetwork: $(network2.subnetwork_self_link)
              nic_type: VIRTIO_NET