Rename /etc/lxc/lxc.conf to /etc/lxc/default.conf.

This will soon be followed by the introduction of a "real" system wide
/etc/lxc/lxc.conf storing global LXC settings.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>