Replace uuidutils.generate_uuid() with str(uuid.uuid4())

uuidutils.generate_uuid() is going to be removed from oslo,
so we need to remove its use in tuskar.

This patch was use str(uuid.uuid4()) to replace
uuidutils.generate_uuid().

Change-Id: I4ac42822fe6269d14abef409a81127ebd4444c8a
Closes-Bug: #1253497
Merge "Replace uuidutils.generate_uuid() with str(uuid.uuid4())"