We don't allow null here sothings would fail if the attribute is missing
in LDAP.

We check that at least one person in LDAP has the specified attribute,
or we error out.

This depends on tugraz specific things, so move it there.

(and fix some deprecation warnings uncovered by this)

Check if the LDAP connections works

The method is gone from the interface

So the user can decide between SSL/TLS.
We still default to encryption on and use the default ports since
no one required something different until now and
to not bloat the config.