Bring Ldap->member_attribute back (bnc#889556)

* At some point, reading of ```/etc/ldap.conf``` was moved away from Ldap module. Since then ```Ldap->member_attribute``` always returns an empty string. But this value is still queried in several points of ```yast-users```.
* This PR fixes the problem by implementing a new agent to read ```/etc/ldap.conf``` and making ```Ldap->member_attribute``` work again as expected.
* It works but I'm not sure if it's the correct fix. Probably ```member_attribute``` should be moved to ```yast-auth-client``` reflecting the change in ```yast-users```. I'll appreciate advise from @varkoly or someone else in that regard.