Adjust debian/debian-edu-config.fetch-ldap-cert. (Closes: #934380)

Use independent conditions to make sure that the LDAP server certificate
is only downloaded once for both host and LTSP chroot.

Add code to validate the LDAP server certificate in case the Debian Edu
RootCA certificate is available for download.

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