add server selection to VPN invite

cluster/models.py

@@ -57,6 +57,10 @@ class Worker(models.Model):
     def __str__(self):
         return self.name
 
+    @property
+    def server_address(self):
+        return self.hostname or self.ip_address or ''
+
     @property
     def display_name(self):
         cluster_settings = ClusterSettings.objects.first()
@@ -98,7 +102,6 @@ class Worker(models.Model):
             return False
 
 
-
 class WorkerStatus(models.Model):
     worker = models.OneToOneField(Worker, on_delete=models.CASCADE)
     last_seen = models.DateTimeField(auto_now=True)