Bug#711093: cdrom: Default user is not added to group sudo

[ashka <shellgratuit@gmail.com>]

Package: cdrom
Severity: normal
Tags: d-i

Dear Maintainer,
Although sudo is installed, the main/default user is not added to the sudo group after install, which leads to errors when trying to run a sudo command.
The user has to use su to get root access, which is unexpected since sudo is installed.

-- System Information:
Debian Release: 7.0
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 3.2.0-4-amd64 (SMP w/1 CPU core)
Locale: LANG=fr_CH.UTF-8, LC_CTYPE=fr_CH.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash