Use pwd.getpwuid() to get login

For some reasons why but os.getlogin() is broken on containers :

http://stackoverflow.com/questions/4399617/python-os-getlogin-problem

when a user is not controlling the terminal.

Let's use instead pwd.getpwuid instead of os.getlogin which would work
in any cases, and would make the function safer to run under containers.

Change-Id: I0cc62e3d13b0e89a0ea1c166ea3ac06cfcc931b3