add apt_cdn for template expansion for sources.list

examples/simple/class/DEBIAN.var

@@ -13,3 +13,5 @@ MODULESLIST="usbhid psmouse"
 
 # if you want to use the faiserver as APT proxy
 #APTPROXY=http://faiserver:3142
+
+apt_cdn=http://deb.debian.org

examples/simple/files/etc/apt/sources.list/GCE

@@ -1,6 +1,6 @@
-deb http://deb.debian.org/debian {release} main contrib non-free
-deb http://deb.debian.org/debian {release}-backports main
-deb http://deb.debian.org/debian-security {release}/updates main contrib non-free
+deb {apt_cdn}/debian {release} main contrib non-free
+deb {apt_cdn}/debian {release}-backports main
+deb {apt_cdn}/debian-security {release}/updates main contrib non-free
 
 deb http://packages.cloud.google.com/apt cloud-sdk-{release} main
 deb http://packages.cloud.google.com/apt google-cloud-packages-archive-keyring-{release} main

examples/simple/files/etc/apt/sources.list/GNOME

@@ -1,2 +1,2 @@
-deb http://deb.debian.org/debian stretch main contrib non-free
-deb http://deb.debian.org/debian-security stretch/updates main contrib non-free
+deb {apt_cdn}/debian stretch main contrib non-free
+deb {apt_cdn}/debian-security stretch/updates main contrib non-free

examples/simple/files/etc/apt/sources.list/preinst

@@ -1,7 +1,7 @@
 #! /bin/bash
 
 # replace {release} in a sources.list template with the value of $release
-
+# replace {apt_cdn} with the value of $apt_cdn
 
 # $1 is the class name used
 # $2 is the path to the file copied
@@ -11,7 +11,13 @@ grep -q '{' $2 || exit 0
 
 if [ -z "$release" ]; then
     echo "\$release is undefined. No substitution performed in $2."
-   exit 0
+else
+    sed -i -e "s/{release}/$release/g" $2
+fi
+
+if [ -z "$apt_cdn" ]; then
+    echo "\$apt_cdn not defined. not substituting in $2."
+else
+    sed -i -e "s#{apt_cdn}#$apt_cdn#g" $2
 fi
 
-sed -i -e "s/{release}/$release/g" $2