Vagrant box creation needlessly downloads Linux from debian-security

The bootstrap installs Linux from Jessie, which we can’t do much about. Then the upgrade downloads an updated kernel from debian-security, which will be unused, since we install Linux from jessie-backports later on.

We should instead install the backports kernel immediately after apt-get update, then purge the jessie kernel, and finally upgrade. This would save quite some time on slow connections when debian-security has a newer kernel than stable.

Similarly, the “Installing Tails build dependencies” bits could be run before apt-get -y dist-upgrade, so we would never upgrade anything twice :)

This matters more now that we will be creating Vagrant boxes locally much more often than before.

What do you think?

Feature Branch: bugfix/12529-simplify-kernel-install-in-vagrant

Parent Task: #5630 (closed)

Original created by @intrigeri on 12529 (Redmine)