Preserve Chef cache between converge runs.

This commit targets specfic directories under the kitchen root for
deletion bewteen runs, specifically:

* data_bags
* roles
* environments
* cookbooks

Note that for chef_zero mode this preserves the `clients/` and `nodes/`
directories so that multiple recoverges will benefit from prior node
data. This is either clever or dangerous, so be aware :)

Closes #237