Use the master constraint again in tox.ini

WHile releasing the new tag for Tempest, we modified the
constraint in tox to be used of stable/ussuri
- https://review.opendev.org/#/c/720578/

Now new tag is released and we can bring back the master
constraint in tox.ini so that Tempest master us master constraint.

Change-Id: Ie17d4830cac1902371123c8acb4d3106ba2fd300
1 file changed