allow custom hp_service_type

The private class method [`.get_endpoint_from_catalog`](https://github.com/fog/fog/blob/master/lib/fog/hp.rb#L200) in `hp.rb` depends on `options[:hp_service_type]` which was hardcoded "Object Storage", as can be seen [here](https://github.com/fog/fog/blob/master/lib/fog/hp/storage.rb#L268). 

For my setup to work, I need to be able to override this option. So I added `:hp_service_type` to the recognizable options and made "Object Storage" an overrideable default. This also seems in line with what was presumably the purpose of the `hp_service_type` option in the first place.