ndb - only run ndb_config test when using "default" config

mysql-test/t/ndb_config.test:
  only run ndb_config test when using "default" config

mysql-test/t/ndb_config.test

 -- source include/have_ndb.inc
+-- source include/ndb_default_cluster.inc
 -- source include/not_embedded.inc
 
 --exec $NDB_TOOLS_DIR/ndb_config --no-defaults --query=type,nodeid,host 2> /dev/null