Fixes #7032 - db backup tasks accept destination file

This is meant to facilitate https://github.com/theforeman/theforeman.org/pull/245
Not sure I like having it in ~, that depends on which user is executing the task (which is 99% likely to be foreman, but still...). I think a relative path to the rake task is better, perhaps File.expand_path('../../../db',__FILE) ?