Fixed attribute passing for add_interface

Server model has an `add_interface` method. This method allows attributes to be passed so you can configure options on the interface. Unfortunately it doesn't pass these options to the create_interface call. This in turn leaves `attributes` empty for `create_vm.rb#create_interface` and subsequently `create_vm.rb#create_nic_backing`. When you define a `DistributedVirtualPortgroup` as the network attribute, and `create_nic_backing` doesn't have `attributes[:datacenter]`, you will end up with a newly created StandardPortgroup instead of the correct `DistributedVirtualPortgroup` in your Backing info for the interface. To solve this you will need to pass along the `datacenter: 'name-your-dc-here'` param when you call server.add_interface, along with `type`, and `network`. 

Tested in ESX 5.5