Always revert to inital state in fake mode

* when nailgun runs in fake mode when node that belongs to cluster and
  is offline will not be completely remove when reset/delete action
* add new option `restore_offline_nodes` to `fake_tasks` decorator
  in tests to control this behavior
* fixed WebTest warning about body in DELETE request in tests from
  test_cluster_handler.py
* add test for new behavior

Change-Id: I9d400e1e8c385cd594c74d7f529e4e1fbe6d7843
Closes-Bug: #1335133