[useradmin] Import users from LDAP subgroup

Recursively import users from LDAP subgroup.

Configurable object class filters for groups and users:
'desktop.ldap.groups.group_filter', 'desktop.ldap.users.user_filter'.
Also, the attribute that is matched against is configured for
groups and users as well: 'desktop.ldap.groups.group_name_attr',
'desktop.ldap.users.user_name_attr'.

The convention is to have child entries be "subgroups"
and use the 'member' attribute to reference members in a group.