Merge lp://qastaging/~mwhudson/livecd-rootfs/bug-1711735 into lp://qastaging/~ubuntu-core-dev/livecd-rootfs/trusty-proposed

Turns out this is super simple, just copy the fix from trunk.

Testing is a bit more involved of course.

I created my own copy of the ~cloud-images-release-managers/+livefs/ubuntu/trusty/docker-ubuntu-core livefs at https://launchpad.net/~mwhudson/+livefs/ubuntu/trusty/docker-ubuntu-core-tests, uploaded a version of livecd-rootfs with no changes to ppa:mwhudson/scratch, built a amd64 and armhf image, checked they were using the right livecd-rootfs, uploaded a version of livecd-rootfs with the change from this branch to the same ppa and built amd64 and armhf again.

I then diffed the pre and post fix tarballs and confirmed that there are no meaningful changes in amd64 ("http://security.ubuntu.com/ubuntu" has become "http://security.ubuntu.com/ubuntu/" in the sources.list) and that the changes we wanted to see are in the armhf tarballs. I also tested apt update in the armhf tarballs via qemu-arm-static and it now succeeds.

If you want to check my working I've uploaded the pre and post fix tarballs to http://people.canonical.com/~mwh/bug-1711735/