Moving mysql password to env variable

[GeNiuS69]

Moving mysql password to MYSQL_PWD variable.
It allows increase security level and prevent warning about cli password: mysqldump: [Warning] Using a password on the command line interface can be insecure.

[elthariel]

I don't like this way of setting env vars on subprocess. But it doesn't seem like Open4 provides another way.

WDYT @tomash ?

[tomash]

An env var is way better than a (potentially logged!) command-line argument. So in general I'm +1 for this PR, as soon as it comes with appropiate documentation.

Moving forward, we should handle secrets in env vars via nowadays-standard https://github.com/bkeepers/dotenv so that they can still be loaded from a file that's never versioned.