lxc-ubuntu-cloud: pass --numeric-owner and -p to untar

Just following up here, Serge raised the question of whether or not the
other two invocations of 'tar' in this script need '--numeric-owner'.
They probably should have it, although its of little concern because the
'build_root_tgz' path is only taken if there is no '-root.tar.gz' file for
download, and the only supported ubuntu release without the -root.tar.gz
download is 10.04 at this point.

Anyway, below is a more complete diff, also including a fix as
'--numeric-uid' is not a valid option to tar.  The name is
'--numeric-owner'.

Signed-off-by: Scott Moser <smoser@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>