You may want to check following things:
1. cat /var/www/wp-config file to see what IP/name is getting configured for DB host and ensure it is reachable at DB port
2. since it is single tier deployment, you can try configuring 127.0.0.1 instead of $CliqrTier_mysql_2_PUBLIC_IP in your web-pre-start script. If this works then you need to check what is returned for this variable during deployment
3. Try disabling iptables/firewall to make sure port is not blocked.