Add LDAP server certificate to the initial LTSP NBD image. (Closes: #932828)

etc/ltsp/ltsp-build-client.conf: Don't create the image by default.
cf3/edu.cf: Define new class 'ltspimages'.
cf3/cf.finalize: Add code to include the LDAP server certificate for all
possible use cases, to generate the image and to adjust various rights.

Signed-off-by: Wolfgang Schweer <wschweer@arcor.de>