Added computed/sane defaults!!

Allows for computed defaults for a select list of pre-determined
platforms.

Currently a .kitchen.yml file of the following would work:

    ---
    driver_plugin: ec2
    driver_config:
      # ...

    platforms:
      - name: ubuntu-10.04
      - name: ubuntu-12.04
      - name: ubuntu-12.10
      - name: ubuntu-13.04
      - name: centos-6.4
      - name: debian-7.1.0

    suites:
      - name: default
        run_list:
          - recipe[something]