python3: Fix UserString import
python3: Fix UserString import

In python3 UserString got moved to collections. This results in the
following traceback while running with python3:

ImportError: No module named 'UserString'

Change-Id: Ica44cd027e012e13828c0a84b3b36607cc92fe69
Signed-off-by: Chuck Short <753a8074f04ffd62a8fff7516e3ae689be8e35ad@canonical.com>

JioCloud/oslo-incubator,JioCloud/oslo-incubator,JioCloud/oslo-incubator