Fixes #7032 - db backup tasks accept destination file

This is meant to facilitate https://github.com/theforeman/theforeman.org/pull/245
As per above, i think this should be:

```
backup_dir = File.expand_path('../../../db', __FILE__)
backup_name = ENV['destination'] || File.join(backup_dir,"foreman.#{Time.now.to_i}.sql")
```