lxc-debian: Fix default mirrors

Fix a typo in the lines inserted in the default sources.list.
Change the default mirror to http.debian.net which is (supposedly) more
accurate and better than cdn.debian.net for a generic configuration.
Use security.debian.org directly for the {release}/updates repository.

Signed-off-by: Gu1 <gu1@aeroxteam.fr>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>