unattended_upgrades: Fix matching security archive

The default configuration we were writing for Debian was only working
for Squeeze, from Wheezy and onwards this wasn't working anymore. This
has to do with the fact that we should now be using Origins-Pattern
according to the unattended-upgrades docs. However, Ubuntu didn't
entirely get with the program yet...

This change reflects the defaults that unattended-upgrade installs on
every platform we support. In order to do so the unattended-upgrades
Debian archive for Squeeze, Wheezy, Lucid, Precise and Trusty were
downloaded and the default /etc/apt/apt.conf.d/50unattended-upgrades
checked for its content with regard to using Allow-Origins or
Origins-Pattern.

Fixes #277
Merge pull request #286 from apenney/fix-for-post-wheezy

unattended_upgrades: Fix matching security archive