AProx integration improvements

- improve uninstall logic to remove appended lines from files like fstab and the davfs2/secrets file
- reverse-proxy aprox via the httpd instance and start aprox with '/aprox' context path to make it proxy cleanly with minimal configuration / risk of transcription errors
- setup both koji and root users' settings.xml files to point to the generated settings for the group CIx-loop on aprox
- mount the stores WebDAV resource, which allows browsing aprox stores as they appear to a process like maven (may require some improved config for davfs2 to get caching right)