KAFKA-18959 increase the num_workers from 9 to 14 (apache#19274)

[JIRA](https://issues.apache.org/jira/browse/KAFKA-18959)


This PR adjusts the default node count for system tests since some tests
are using more than 9 nodes.

This change ensures such test cases can be run successfully without
manual configuration.

Reviewers: Chia-Ping Tsai <[email protected]>

Author: apalan60

vagrant/system-test-Vagrantfile.local

@@ -18,7 +18,7 @@
 # it to Vagrantfile.local
 num_zookeepers = 0
 num_brokers = 0
-num_workers = 9
+num_workers = 14
 base_box = "kafkatest-worker"