You may need to create this file, otherwise edit the existing one: /etc/mysql/conf.d/my.cnf (in most cases this is the correct location)
Reference back to your MySQL database hostname, user, and password that were setup.
[client]
host=yourdb.cluster-xxxx.rds.amazonaws.com
user=youruser
password="supersecret"
These values will then be used during creation of a new site