implement user creation in osusers.admin

- add osusers.admin.UserCreationForm
- add dummy osusers.tasks implementation with create_ldap_group and
  create_ldap_user
- fix UserManager.get_next_username
- add proper transaction handling in UserManager.create_user
- add calls to create_ldap_user and create_ldap_group to
  UserManager.create_user

gnuviechadmin/osusers/tasks.py

@@ -0,0 +1,13 @@
+from __future__ import absolute_import
+
+from celery import shared_task
+
+
+@shared_task
+def create_ldap_group(group):
+    return group
+
+
+@shared_task
+def create_ldap_user(user, password):
+    return user, password