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 
Theoretically it's directory, so when there's no one already created it'll create one for you. On the other hand if you have already such file it'll be mounted as a file. We're already doing this a couple of lines before see [util.go#L13](https://github.com/openshift/origin/blob/master/pkg/build/strategy/util.go#L13).