Add Cloud Networks capabilities

Allows for the addition of isolated Cloud Networks to instances. By default, instances will have a public and ServiceNet network available. ServiceNet can be removed by passing `:no_servicenet` to the network configuration.

```ruby
config.vm.provider :rackspace do |rs|
  rs.username = "mitchellh"
  rs.api_key  = "foobarbaz"
  rs.network '443aff42-be57-effb-ad30-c097c1e4503f'
  rs.network '5e738e11-def2-4a75-ad1e-05bbe3b49efe'
  rs.network :no_servicenet
end
```

The syntax for this option uses a `network` method instead of assignment because we'll need to accept additional arguments once the `v4-fixed-ip` option is introduced to Rackspace Cloud Networks. Also, this style is more in-line with the `config.vm.network` syntax. I'm not necessarily in love with it, so let me know if anything needs changed.