[Tempest] Tag stein-last

Stein branch is in Extended Maintenance now[1]. All Tempest
plugins are branchless which means master version of Tempest
and its plugins is used to test the supported stable branches.

Once stable branch is moved to EM state then, Tempest and its
plugins compatible tag needs to be released so that we can
keep testing the EM stable branches with this tag once master
Tempest and its plugins are not compatible[2].

Documenting this new tag name here
- https://review.opendev.org/c/openstack/project-team-guide/+/769821

This tag will help for testing the stable/stein with single
compatible tag of Tempest and its plugins.

More detail on compatible version policy for Tempest and its
plugins is here
- https://docs.openstack.org/tempest/latest/tempest_and_plugins_compatible_version_policy.html

[1] https://releases.openstack.org/stein/index.html
[2] https://docs.openstack.org/tempest/latest/stable_branch_support_policy.html

Change-Id: I4a8e162442c84ab49c0654ef3264bcc9e74f1ae2

deliverables/stein/tempest.yaml

@@ -10,6 +10,10 @@ releases:
       - repo: openstack/tempest
         hash: a955095f8369e2e15c97a536edc1a9c077208129
     version: 20.0.0
+  - version: stein-last
+    projects:
+      - repo: openstack/tempest
+        hash: f9d31f2c46204abbbef53d3c0ae0f80125878fc9
 repository-settings:
   openstack/tempest:
     pypi-name: Tempest