Reliable way to detect username

Note: Docker doesn't expose $USER as a environement variable

Change-Id: I4f81dbc11401bf2eea45546463c110e6b9da5d47
Merge commit 'refs/changes/27/2427/4' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  Reliable way to detect username
This is Tuleap 7.0.99.25

Change-Id: I05f7aea1c4b99c8283fe1995f3f4af6f09ac22cb