Split the base box into one used for dev and one for the tutorial.

Fixes #509.

TODO:
 1. [ ] Update the documentation. (It will be the same as currently, but for both of the boxes)
 2. [ ] Update the release process to generate a new tutorial image with the just released packages.
Since it was a two-line change, I've set the dev box to use the zfs-testing repo. This will pull the latest version whenever the box is built, or when `yum update` is run inside the image.