If you do not provide these keys, they will be set to the defaults in the groupvars/all/vars.yaml file under each role. For production deployments, you may want to generate your master and join keys and to apply them to all the nodes using the following command.
MASTER_KEY_VALUE=$(openssl rand -hex 32) JOIN_KEY_VALUE=$(openssl rand -hex 32) ansible-playbook -vv platform.yml -i hosts.ini --extra-vars "master_key=$MASTER_KEY_VALUE join_key=$JOIN_KEY_VALUE"
Note
Remember to save the generated master and join keys for future upgrades.