Openstack provider: Temp url support for Files

The PR aims at providing a common interface for creating temporary urls from files. The additions are corresponding to the HP provider temporary url methods. 
We should be able to create temporary urls from File objects. The PR keeps the get_object_https_url function to keep the OpenStack provider backwards compatible. It just adds the temp url method generation methods according to the HP provider to guarantee a common interface for Openstack providers.

This pull requests adds the #temp_signed_url function to the Fog::Storage::Openstack::File class for generating temporary urls.

In addition it refactors the get_object_https_url request:
- addition of the #generate_object_temp_url method
- refactoring of the #get_object_https_url method -> extract body to private function (#create_temp_url)