backends: add new settings for LDAP servers: attributes, attribute_mappings and attributes_mandatory_values
backends: add new settings for LDAP servers: attributes, attribute_mappings and attributes_mandatory_values

The setting 'attributes' is a list of attribute to retrieve from the
user record.

The setting 'attribute_mappings' gives rule to map some attributes
values to another attribute, it's a list of pair of attribute names:
(from_attribute, to_attribute).

The setting 'attributes_mandatory_values' force some values for some
attributes. It's a dictionary {attribute_name: [attribute_value1,
attribute_value2, ...]}.

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