Fix random username & password generation

[stuft2]

There are some constraints around what a username and password can be for an RDS database. It varies between the database families but I'm sure there is a common set of constraints we can follow to avoid failed deployments. For example, the username must follow these constraints:

• It can contain 1–16 alphanumeric characters and underscores.
• Its first character must be a letter.
• It can't be a word reserved by the database engine.

The docs describing the constraints are here: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_CreateDBInstance.html