Adjust share/debian-edu-config/d-i/finish-install: (Closes: #941574)

Use 'dpkg-reconfigure -u --no-reload debian-edu-config' to add post-up
stanza to /etc/network/interfaces eth0 entry conditionally.

Signed-off-by: Wolfgang Schweer <wschweer@arcor.de>
Signed-off-by: Holger Levsen <holger@layer-acht.org>