TT#69603 jessie-backports no longer exists, adjust apt sources.list

The jessie-backports repository no longer exists, see
https://lists.debian.org/debian-devel-announce/2019/03/msg00006.html

Our base AMI for jessie (ami-402f1a33, being the most recent available AMI for
Debian/jessie) is no longer maintained by Debian and ships a
/etc/apt/sources.list.d/backports.list file that breaks apt usage:

| $ cat /etc/apt/sources.list.d/backports.list
| deb http://cloudfront.debian.net/debian jessie-backports main
| deb-src http://cloudfront.debian.net/debian jessie-backports main
| [...]
| $ sudo apt-get update
| W: Failed to fetch http://cloudfront.debian.net/debian/dists/jessie-backports/main/source/Sources  404  Not Found [IP: 99.86.122.86 80]
|
| W: Failed to fetch http://cloudfront.debian.net/debian/dists/jessie-backports/main/binary-amd64/Packages  404  Not Found [IP: 99.86.122.86 80]
|
| E: Some index files failed to download. They have been ignored, or old ones used instead.

So if file /etc/apt/sources.list.d/backports.list exists and contains
"jessie-backports" then remove the file.

Change-Id: Idd4b6b376c2e732fa52bd11555ea92971cf52c61

ec2-create-ce

@@ -371,6 +371,11 @@ wget -O /tmp/ngcp-installer.deb ${INSTALLER_URL}
 dpkg -i /tmp/ngcp-installer.deb
 sed -i 's/cdn-aws.deb.debian.org/deb.debian.org/' /etc/apt/sources.list
 
+if [ -f /etc/apt/sources.list.d/backports.list ] && grep -q 'jessie-backports' /etc/apt/sources.list.d/backports.list ; then
+  echo "jessie-backports no longer exists, removing /etc/apt/sources.list.d/backports.list"
+  rm -f /etc/apt/sources.list.d/backports.list
+fi
+
 if [ "$SYSTEMD_SUPPORT" = "false" ] && [ -d /run/systemd/system ] ; then
   echo "Switching from systemd to sysvinit (pre-reboot)"
   apt update