Merge pull request #1321 from wallyworld/lp-1328958

Change the way the ubuntu user is detected

Fixes https://bugs.launchpad.net/ubuntu/+source/juju-core/+bug/1328958

Instead of looking for /home/ubuntu, we grep the passwd file to determine if the ubuntu user exists.

(Review request: http://reviews.vapour.ws/r/640/)

(Review request: http://reviews.vapour.ws/r/642/)