test(mocker): added head_object to mocked swift connection
storage(driver): avoid downloading blob by using head_object

Using head_object makes the swift driver a lot faster by avoiding
unecessary downloads in get_size and exists methods.
storage(driver): implement content_redirect_url method

With this method defined, if storage_redirect is set to true the request
to the layer will only return a 302 redirect to a swift URL. This allows
the client to download the image directly from swift instead of blocking
the registry.
storage(driver): allow explict object_storage_url config

This allows you to set a different object_storage_url than the one
returned by the auth request.