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.
@bparees just like @ncdc mentioned, we got rid of env variable from the OpenShift core, but you still need to pass it to build somehow, thus this var here.