Allow test-kitchen to use some more memory

and possibly more CPUs as the ioapic is supposed
to allow VirtualBox to use all cores.
Update installation of kafka in source recipe

With the new release I could just copy everything.
Maybe I could before as well, but yeah.
Define checksum variable inline unless statement

Gotta save those lines, yo.
Update Vagrantfile

* Enable vagrant-berkshelf.
* Explicitly add java default recipe.
* Default to kafka source recipe.