Default password for MySQL user

[TheLukaszNs]

The default password for the OpenDFU MySQL user is nowhere documented. The user must check the setup.sh file to see what the password (and username) really is.

We should either print the username and password by the setup.sh when it finishes setting up the database or document these values in the Wiki (maybe installation section would be a good place).

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Also we should use more secure password than '123' 👍