[bug 1335844] unattended-upgrade should only email on error

group_vars/all/unattended_upgrade.yml

@@ -2,3 +2,4 @@ apticron_email: 'hostmaster@mozilla.community'
 unattended_origins_patterns:
   - 'origin=Ubuntu,archive=${distro_codename}-security,label=Ubuntu'
 unattended_mail: 'hostmaster@mozilla.community'
+unattended_mail_only_on_error: 'true'