Ldap extra_attributes are discarded if not in user model

[Jose Castro Leon]

When configuring the ldap driver, there is the possibility to add extra attributes to the user/group objects, this is configured in ldap by the (user|group)_additional_attribute_mapping.

Although the ldap queries retrieve the information from the attribute specified, when this gets then parsed in _ldap_res_to_model it gets ignored as the user model does not contain these attributes

https://github.com/openstack/keystone/blob/master/keystone/identity/backends/ldap/common.py#L1358

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/675303

[Aleksey Myltsev (amyltsev)]

Our use case for extra attributes:
We would like to notify users if their project has been affected, for this reason we would like send email on their distribution group. We want add property email for groups and fill in this field with mail attribute from LDAP.