Pad out the segment identifier for Swift segments and change test back to string comparison

Makes us follow the docs closer

storage/test/test_swift.py

@@ -84,7 +84,7 @@ class FakeSwift(object):
         if ('container-name/' + key).startswith(prefix):
           new_contents.append((key, value['content']))
 
-      new_contents.sort(key=lambda value: int(value[0].split('/')[-1]))
+      new_contents.sort(key=lambda value: value[0])
 
       data = dict(data)
       data['content'] = ''.join([nc[1] for nc in new_contents])