Added recursive management of minion.d and master.d.

This allows users to easily add configuration settings that the formula doesn't yet provide, without having to modify the existing templates. They do this by putting them in an extra file under salt/files/master.d or minion.d. 

In my case I wanted to be able to set Mine settings, which aren't in the templates. I could easily see someone else needing it; for example, because of a gap between a master or minion setting being added to Salt, and the same setting being added to this formula. 

The underscore in the new file name is there because salt appears to load the .d directories alphabetically with later files overriding earlier ones. This way, settings in user-created files will override the ones we provide (although they should really be using pillar if they want to do that, so in theory it shouldn't be an issue). 

Clean recurse seems like the right thing to me here, but I'm not 100% sure of that. Opinion?