First draft of using the overrides

This commit contains a first draft of using "overrides". There were two possible approaches:

- white listing what we want to explicitly override
- recursively comparing what we have on the override with the destination, and linking it to there

I've chosen the first one, as it's simpler to implement and we could use this idea to start thinking if we want to do the recursive approach. 

Please, do *not* merge this commit. I'll make some changes to the README.md file, to include more examples. Also, if this is the way to go, I'll add the users/roles/groups.properties to the mix. 
For preview -- look at the `base` branch: https://github.com/jboss/dockerfiles/tree/base