Workaround the insecure password warnings emitted by MySQL 5.6

When using MySQL 5.6, the following output is emitted every time a backup is created, even with CRON=1.

```
Warning: Using a password on the command line interface can be insecure.
```

This fix works around that by sending the password via the [`MYSQL_PWD`](http://dev.mysql.com/doc/refman/5.6/en/environment-variables.html) environment variable.

See merge request !1139