Revert "repart: Ensure files end up owned by root in generated filesystems"

Revert "repart: Ensure files end up owned by root in generated filesystems"

This reverts commit e59678b2cf42e4206ddabc959d3cf9a5a865ecdc.

We also modify the repart integration tests to make them pass with the
changes in this commit. In short, we have to make sure every file is
owned by the user executing repart. We use tee instead of cat since it
makes that easier. This also has the benefit of improving debugability
as seeing the config file contents on stdout makes it easier to know
which test is failing.