Bug 1150921 - Problem with pushing images to public repo caused by missing .dockercfg file in $HOME.

It appears that recently `fedora:20` image changed it's `$HOME` setting. Since we plan to move to `centos7` image where also `$HOME` is set to `/root` I've decided to go this way instead of hardcoding the location. This way we'll avoid changing the location on every image/`$HOME` variable change. I'm open to discussion, though. 
This change is another requirement for #143.
Hmmm... I was talking about that with @bparees yesterday and we weren't sure about that idea. Currently after all the R&D around images I've done, it does makes sense to have it set to either `/root` or `/`. I'm pretty sure the 1st would be preferred then. @csrwng @ncdc is it safe assumption to do so for STI as well?