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 
If a BuildInput doesn't specify Registry, but the minion has .dockercfg, this would result in a `docker push` - is that the behavior we want?