[Feature request] Allow to get secrets (passwords) from environment vars

[pataquets]

To avoid secrets into .conf files which might then be commited, having env vars as the default password if empty (instead of '') would do the trick. I will happily work on it and submit a PR, but since I'm also looking forward for #18, the specific env var name might be MYSQL_[INSTANCENAME]_PASSWORD or something similar, but feedback is welcome. Thanks in advance.