fix(nodes list in email): not print nodes list in the email if more t…

…hen 20 nodes

There are scale tests with a lot of nodes. It will be so big list of nodes to print it in the
email. Don't print nodes info if test cluster has more then 20 nodes (live and terminated)

sdcm/tester.py

@@ -2750,7 +2750,12 @@ def get_email_data(self):  # pylint: disable=no-self-use
         return {}
 
     def all_nodes_scylla_shards(self):
-        all_nodes_shards = defaultdict(list)
+        all_nodes_shards = defaultdict(list, {'live_nodes': [], 'dead_nodes': []})
+
+        # There are scale tests with a lot of nodes. It will be so big list of nodes to print it in the email
+        if len(self.db_cluster.dead_nodes_list) + len(self.db_cluster.nodes) > 20:
+            return all_nodes_shards
+
         for node in self.db_cluster.nodes:
             ipv6 = node.ipv6_ip_address if node.ip_address == node.ipv6_ip_address else ''
             all_nodes_shards['live_nodes'].append({'name': node.name,