use f21 image from fedora rel-eng

More info about this image can be found here:
http://koji.fedoraproject.org/koji/taskinfo?taskID=7978938

The tarball available on this page contains the rootfs along with metadata.
Only the rootfs has been untarred and re-tarred into fedora-21-beta.tar.xz

Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>