Add OpenStack security group to platform version.

Hello.  I've added support to specify security groups when creating an OpenStack instance in the platform DSL.

For example:

platform "ubuntu" do
  version "11.04" do
    image_id 1240
    flavor_id 101
    keyname "tpot@localhost"
    security_groups ["basenode"]
    install_chef true
    ssh_user "ubuntu"
    ssh_key "#{ENV["HOME"]}/.ssh/id_rsa"
  end
end