lxc-debian: support stretch (Debian 9) images

Signed-off-by: Antonio Terceiro <terceiro@debian.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>

templates/lxc-debian.in

@@ -534,7 +534,7 @@ fi
 
 current_release=`wget ${MIRROR}/dists/stable/Release -O - 2> /dev/null | head |awk '/^Codename: (.*)$/ { print $2; }'`
 release=${release:-${current_release}}
-valid_releases=('squeeze' 'wheezy' 'jessie' 'sid')
+valid_releases=('squeeze' 'wheezy' 'jessie' 'stretch' 'sid')
 if [[ ! "${valid_releases[*]}" =~ (^|[^[:alpha:]])$release([^[:alpha:]]|$) ]]; then
     echo "Invalid release ${release}, valid ones are: ${valid_releases[*]}"
     exit 1