use primitive fields instead of models for tasks

- modify osusers.tasks and osusers.models to avoid serialization of full
  models for celery tasks and use the required fields only

gnuviechadmin/osusers/tasks.py

@@ -14,13 +14,13 @@ class LdapRouter(object):
 
 
 @shared_task
-def create_ldap_group(group):
-    return group.groupname
+def create_ldap_group(groupname, gid, descr):
+    pass
 
 
 @shared_task
-def create_ldap_user(user, password):
-    return user.username
+def create_ldap_user(username, uid, gid, gecos, homedir, shell, password):
+    pass
 
 
 @shared_task
@@ -34,7 +34,7 @@ def remove_ldap_user_from_group(username, groupname):
 
 
 @shared_task
-def delete_ldap_user(user):
+def delete_ldap_user(username):
     pass