The LDAP backend supports mapping between LDAP and keystone user
attributes via the 'user_<attribute_name>_attribute' settings in the
LDAP driver configuration.
The current implementation is incomplete, since there is no support for
specifying a 'user_description_attribute' setting for user get (read)
This change adds support to the LDAP backend for mapping of user
description attributes via a 'user_description_attribute' configuration
also during user retrieval.