Allow custom hp_service_type [swift]

For our setup to work, we need the `hp_service_type` to be customizable. This PR makes it possible to define it in your bosh manifest. This PR depends on a corresponding PR for the source at anynines [[link](https://github.com/anynines/cf_swift_service/pull/3)].

Therefore I have added `"Object Storage"` to the example manifest as the default value for `hp_service_type`. This corresponds with the default that already existed in `Fog::Storage::HP` [[link](https://github.com/fog/fog/blob/master/lib/fog/hp/storage.rb#L324)]. The fix is backwards compatible, the option can be left out of the manifest without any troubles.