stop chowning everything

[Paul Collins]

```python
def configure_directories(conf, hostname):
    if not os.path.isdir("/srv/ftp.root"):
        mkdir("/srv/ftp.root")
    chownr("/srv/ftp.root", conf.mirror_user(), conf.mirror_user(), follow_links=False)
```

This seems unnecessary and, as we host more and more content on these machines, upgrades become slower and slower.

The charm already creates the root of the mirror path with the correct ownership, and the sync scripts run as that user.

Finally, chownr does not even change the ownership of the path from which it begins its recursive chown.