Add celery tasks for ldap synchronisation

[t-book]

Following should be done:

• add tasks.py to ldap contrib app which run management commands for keeping users in sync
• add the tasks to beat schedule in geonode project
• add the settings under the ldap conditional block: https://github.com/GeoNode/geonode-project/blob/master/project_name/settings.py#L138

[t-book]

Current status:

1. Created tasks for geonode_ldap: https://github.com/csgis/geonode-contribs/blob/ldap_celery_tasks/ldap/geonode_ldap/tasks.py

2. Updated Settings of geonode-project: https://github.com/csgis/geonode-project/commit/d786f698b484bb2a760f1e3d6be51ebffa89b670#diff-4f4c55450c39ce510cb2d3646b9b3301

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:heavy_check_mark: Management commands run from shell work fine :heavy_check_mark: updateldapgroups management command run by celery beat task works fine :heavy_multiplication_x: updateldapusers management command run by celery beat tasks raises following error https://hastebin.com/eviyelocev.log . My guess is there is some conflict with sqlite haven't tested with postgis backend yet

[afabiani]

It's a specific sqlite3 error, I would ignore it. It won't happen using postgresql