Add --provisioner to `kitchen init` to override default, chef_solo.

Now the implicit default is explcit in the generated .kitchen.yml. The
current starting file (assuming vagrant and metadata.rb) is:

    ---
    driver: vagrant
    provisioner: chef_solo

    platforms:
      - name: ubuntu-12.04
      - name: centos-6.4

    suites:
      - name: default
        run_list:
          - recipe[mycookbook::default]
        attributes: