add daemon user to required groups

bareos storage daemon user must be in groups tape and/or disk
to be able to access tape devices.
Due to different behavior of different distributions
(install order if not always the same),
every package that requires a specific group/user set this up on its own.

preinstall:
  bareos-common: setup default daemon group bareos and user bareos
  bareos-filedaemon: setup fd group (bareos) and user (root)
  bareos-storage: setup sd group (bareos) and user (bareos)
  bareos-director: setup dir group (bareos) and user (bareos)

postinstall:
  bareos-storage:
    call bareos-config setup_sd_user, which
    checks if sd group and user exists, otherwise it creates them,
    and add the sd user (bareos) to the groups tape and disk, if they exists.

Tested on: Debian 6, Ubuntu 12.04 (32bit), SLES11SP2, Centos5

Fixes #99: user bareos unable to operate tape changer due to wrong permissions

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>