Allow specifing docker registry credentials - card #338

This is initial commit with docker registry credentials. I'd like to hear comments whether it's better this way or to create a separate object that holds all this information? Obviously, it's only temporary solution for demo etc, and will be replaced by more generic one. Still extensive readme ahead of me.
@mfojtik @smarterclayton @csrwng 
Agree and agree

----- Original Message -----
> > @@ -16,6 +16,6 @@ fi
> >  
> >  docker build --rm -t $TAG $DOCKER_CONTEXT_URL
> >  
> > -if [ -n "$DOCKER_REGISTRY" ]; then
> > +if [ -n "$DOCKER_REGISTRY" ] || [ -s "/.dockercfg" ]; then
> 
> I think we need to get rid of the $DOCKER_REGISTRY environment variable, as
> it limits the build system to a single target registry per minion. It was
> originally coded this way because it was a POC, and I didn't know any better
> :-)
> 
> I also think our builds should always push to a registry, as they're not very
> useful otherwise.
> 
> ---
> Reply to this email directly or view it on GitHub:
> https://github.com/openshift/origin/pull/116/files#r17972451