TurnKey AMIs don't have a default root password. By default, it's random. The Hub let's you optionally set this but if you're launching directly from AWS management console or cli tools (as opposed to the TurnKey Hub), you can find the random root password printed to the instance's virtual console, towards the end. AWS management console has an option to show the instance's console output.
An alternative might be to just log in with your SSH keys and then change the root password needed for webmin access
I realize this is inconvenient (if you don't use the Hub) but setting the root password to a fixed default would be very dangerous from a security standpoint as it would open a window of vulnerability between the time you launched an instance to the time you realized you had to change the password. Using automated tools it doesn't need to take more than a few seconds for an attacker to find and compromise your machine before you know it.
The default root password is random (for security purposes)
An alternative might be to just log in with your SSH keys and then change the root password needed for webmin access
I realize this is inconvenient (if you don't use the Hub) but setting the root password to a fixed default would be very dangerous from a security standpoint as it would open a window of vulnerability between the time you launched an instance to the time you realized you had to change the password. Using automated tools it doesn't need to take more than a few seconds for an attacker to find and compromise your machine before you know it.