story #4523 - Remove 'nobody' from permission list

So we can not lock ourselves outside an artifact

Nota: No check on SOAP api, as it's not yet possible to
update permissions via soap according to the doc

Change-Id: I63052ace5938e31b5f25bbe995e8d8e4835223ac
Merge commit 'refs/changes/05/705/1' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  story #4523 - Remove 'nobody' from permission list

Change-Id: I63563f3a111c7fc587482249b8c976db624cfca1
This is Tuleap 6.3.99.6

Change-Id: Ic1e191968b0c26a9f723fa92499268d8c538904c