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 
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.