Support insert into table function cluster if the cluster does not have sharding key.

[filimonov]

CREATE TABLE default.x AS system.numbers ENGINE = Log;

INSERT INTO FUNCTION cluster('test_cluster_two_shards', default, x) SELECT * FROM numbers(10)

Received exception from server (version 20.11.1):
Code: 55. DB::Exception: Received from localhost:9000. DB::Exception: Method write is not supported by storage Distributed with more than one shard and no sharding key provided. 

0 rows in set. Elapsed: 0.001 sec. 

INSERT INTO FUNCTION cluster('test_cluster_two_shards', default, x, rand()) SELECT * FROM numbers(10)

Received exception from server (version 20.11.1):
Code: 42. DB::Exception: Received from localhost:9000. DB::Exception: Table function 'cluster' requires from 2 to 3 parameters: <addresses pattern or cluster name>, <name of remote database>, <name of remote table>. 

0 rows in set. Elapsed: 0.001 sec.