test server "zone" property is a map

Test server code in gomaasapi needs to be updated to align with MAAS.

Currently, "zone" property is still treated as field by gomaasi infrastructure and is searched for using matchField(node, "zone", zoneName) instead of getMap.

@see gomaasapi/testservice.go#findFreeNode

So instead of:

   if zoneName != "" && !matchField(node, "zone", zoneName) {
    continue
   }

There should be something like this:

   if zoneName != "" {
    zoneNameObj := maasify(server.client, map[string]interface{}{"name": zoneName, "description": "description"})
    node.GetMap()["zone"] = zoneNameObj
   }