Computed defaults

Allows for computed defaults for a select list of pre-determined
platforms.

Currently a .kitchen.yml file of the following would work:

```ruby
---
platforms:
  - name: ubuntu-10.04
  - name: ubuntu-12.04
  - name: centos-5.9
  - name: centos-6.4
  - name: debian-7.1.0

suites:
  - name: default
    run_list:
      - recipe[something]
```