keystone ldap now allows overloading of username and tenant name

As per https://review.openstack.org/#/c/11737/ - there's now this new ability for ldap-backed keystone, which should be added to the docs:

   633
   634 The default object classes and attributes are intentionally simplistic. They
   635 reflect the common standard objects according to the LDAP RFCs. However,
   636 in a live deployment, the correct attributes can be overridden to support a
   637 preexisting, more complex schema. For example, in the user object, the
   638 objectClass posixAccount from RFC2307 is very common. If this is the
   639 underlying objectclass, then the *uid* field should probably be *uidNumber* and
   640 *username* field either *uid* or *cn*. To change these two fields, the
   641 corresponding entries in the Keystone configuration file are::
   642
   643 [ldap]
   644 user_id_attribute = uidNumber
   645 user_name_attribute = cn