Add Temp-Url-Key arguement for generating CloudFiles TempURL

Since TempURLs can be generated locally without communication with the API, it's redundant and slow for `CloudFilesStorageDriver.ex_get_object_temp_url()` to request the API for the `temp_url_key` during each call.

I propose an optional argument to allow calls this method to to supply the temp_url_key

ex_get_object_temp_url(self, obj, method='GET', timeout=60):
changed to..
ex_get_object_temp_url(self, obj, method='GET', timeout=60, temp_url_key=None):

If supplied, it would replace the logic for requesting the temp_url_key via the API.