Improve benchmark.runner.UserGenerator

Rename a wrong contructor parameter name and also fix the potential bug
coonected with self.tenants in __exit__() that can arise when
UserGenerator.create_users_and_tenants() has never been called.

Change-Id: I0f9beed879c2ee7dadb32fc24af650b8daeacf7e
Merge "Improve benchmark.runner.UserGenerator"