debian/debian-edu-config.fetch-ldap-cert: Retrieve TJENER's PKI server...

debian/debian-edu-config.fetch-ldap-cert: Retrieve TJENER's PKI server certificate only once per host. With the intention of fixedly attaching a Debian Edu workstation to a specific Debian Edu network. This re-introduces the behaviour of fetch-ldap-cert (and thus, of Debian Edu workstations) in stretch and earlier. (Closes: #931413).