[heat-tempest-plugin] Tag xena-last

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

Once any stable branch is moved to EM state then, Tempest and
its plugins compatible tag[2] needs to be released so that we can
keep testing the EM stable branches with that tag once master
Tempest and its plugins are not compatible[3].

Tagging 'xena-last' with the latest hash which is nothing but 1.7.1
tag.

[1] https://review.opendev.org/c/openstack/releases/+/881254
[2] https://docs.openstack.org/tempest/latest/stable_branch_support_policy.html
[3] https://docs.openstack.org/tempest/latest/tempest_and_plugins_compatible_version_policy.html

Change-Id: I85f3a7f66a30789be7d56877a2ed5369a28d1e75

deliverables/xena/heat-tempest-plugin.yaml

@@ -17,3 +17,7 @@ releases:
     projects:
       - repo: openstack/heat-tempest-plugin
         hash: adb8ba7dea4e93414957b72ea9ff5423ba39cb56
+  - version: xena-last
+    projects:
+      - repo: openstack/heat-tempest-plugin
+        hash: a3719a852c4485ec4a1269f6b4acf7e18432365a