add user defined keys for networks/netdev

Every network in the profile and for the nodes can have induvidual keys values
pairs, called Tags, which are propergated to the templates. So they can be used
for things like MTU size like
  `wwct node set node01 --nettag mtu=9000`
and in in ifcfg.xml.ww
```
{{ if $netdev.Tags.mtu -}}
<mtu> {{ $netdev.Tags.mtu }} </mtu>
{{ end -}}
```

internal/pkg/node/modifiers.go

@@ -91,6 +91,12 @@ func (config *nodeYaml) NodeUpdate(node NodeInfo) error {
 		config.Nodes[nodeID].NetDevs[devname].Type = netdev.Type.GetReal()
 		config.Nodes[nodeID].NetDevs[devname].OnBoot = netdev.OnBoot.GetReal()
 		config.Nodes[nodeID].NetDevs[devname].Default = netdev.Default.GetReal()
+		config.Nodes[nodeID].NetDevs[devname].Tags = make(map[string]string)
+		for keyname, key := range netdev.Tags {
+			if key.GetReal() != "" {
+				config.Nodes[nodeID].NetDevs[devname].Tags[keyname] = key.GetReal()
+			}
+		}
 	}
 
 	config.Nodes[nodeID].Tags = make(map[string]string)