Configure the unattended-upgrades

* Enable the -updates repository as well as -security
* Remove unused dependencies after an upgrade
Setup ssmtp and bsd-mailx to send mail via mailgun
Send updates to infrastructure-staff
Move 20auto-upgrades to 10periodic
Autoclean and pre-download packages
rename from auto-security to unattended-upgrades since it's not just security
Merge pull request #3 from python/unattended-upgrades

Unattended upgrades