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

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.