Ask for the 'memberOf' attribute to be returned in the user query only
if 'ldap_load_groups' is set to True.
This fixes the issue that currently LDAP authentication can only be used on
LDAP servers that know this non-standard (it's an Active Directory extension)
attribute.
Other LDAP servers either do not necessarily have the group memberships
stored in the user object (e.g. OpenLDAP), or use different attributes for
this purpose (e.g. Novell eDirectory uses 'groupMembership')
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