LDAP auth: introduce config option 'ldap_groups_attribute'

This attribute is supposed to hold the group membership information
if the config option 'ldap_load_groups' is True.
If not given, it defaults to 'memberOf' for Active Directory.

Introducing this options allows one to use radicale's LDAP auth with groups
even on LDAP servers that keep their group memberships in a different attribute
than 'memberOf', e.g. Novell eDirectory which uses 'groupMembership'.

DOCUMENTATION.md

@@ -941,6 +941,12 @@ Load the ldap groups of the authenticated user. These groups can be used later o
 
 Default: False
 
+##### ldap_groups_attribute
+
+The LDAP attribute to read the group memberships from in the user's LDAP entry if `ldap_load_groups` is True.
+
+Default: `memberOf`
+
 ##### ldap_use_ssl
 
 Use ssl on the ldap connection