I am not aware of any tool that can do this. I guess Nessus would pick up if the issue of default password. I am not sure how you could detect a weak password unless you brute forced the login and found the password to be 12345 or another easy to guess password.
You should be able to write a simple python script to do this. One thing that might be an issue with is how you detect a weak password and then replace it. As if you want the hole process to be automated you would need to store the password in the script in plaintext.
I am not the best programmer in the world so maybe other might have way around this.