Unattended upgrades oldstable for wheezy

Add "oldstable" to the default update origins to ensure
the updates keep working after wheezy+1 gets released
    
See unattended-upgrades 0.79.5+wheezy1 at http://metadata.ftp-master.debian.org/changelogs//main/u/unattended-upgrades/unattended-upgrades_0.79.5+wheezy1_changelog
and the referenced bug report at https://bugs.debian.org/711826