Better guessing of username.
Better guessing of username.

Also check USER env variable, and prompt if not found.

Change-Id: I91327a7869be2de6f0e5586142c517e13c64cb49

openstack-infra/git-review,entropia1/git-review,jeeloo/git-review,meelapshah/git-review,meelapshah/git-review,ka2m/git-review