Various improvements making the cookbook more flexible

Although I've added some property key for a SQL back-end, I have not attempted to address the "Provisioning a SQL database" steps mentioned at http://docs.datomic.com/storage.html.

So, at present, the cookbook alone will not get Datomic ready for a SQL system. I'll be doing this process myself soon, so I'll see what happens and possibly push some things back to this cookbook.