Sometimes, LDAP organization is such that groups membership cannot be computed
with username only. User DN is required to retrieve groups.
e.g. user Joe has a username joe and a cn of Joe Blogs, resulting in a dn of
cn=Joe Blogs,ou=users,dc=example,dc=com which is needed to retrieve groups
but cannot be computed from joe only.
Issue was reported in issue #146
Previously, string "{0}" was replaced by the user dn in the groups_filter
attributes of the LDAP configuration.
However, if the groups children only have a memberUid attribute, one would
like to use the username instead of the user dn.
Since the user dn can be built from the username, "{0}" is now replaced
by the username instead of the user dn so that an LDAP relying on attribute
'memberUid' can be used.
