Ensure RabbitMQ state never leaves a user with a password of "None"

The RabbitMQ add user command requires a password, currently, we can pass None (as in nil/null) in here, and the rabbitmq module will cast to the string "None".

Here, we ensure that when we're going to clear the password immediately after creating
the account, we generate a random password. This ensures we don't have an account with
"None" as the password should salt or rabbitmq fail between creating the user, and
clearing the password.