Can't add new node to the cluster if I don't specify data-center explicitly.

[nyh]

In materialized_views_test.py I wanted to add a test doing something like:

        session = self.prepare(rf=rf)
        session.execute("CREATE TABLE t (id int PRIMARY KEY, v int)")
        node4 = new_node(self.cluster)
        node4.start(wait_other_notice=True, wait_for_binary_proto=True) 

This did not work... The fourth Scylla instance failed to start up. The problem was that the file
/home/nyh/.dtest/dtest-kawO4S/test/node4/conf/cassandra-rackdc.properties instead of containing

dc=dc1
rack=RAC1

as was expected, it contain a comment with explanation, but no content, apparently copied from the default Cassandra configuration.

If I change the code to explicitly specify the data center:

        node4 = new_node(self.cluster, data_center='dc1')

Everything works.