zuul: pin cinder-tempest-plugin/stein-last on stein

Right now master is used, but it is not compatible with stein.
Tempest is already pinned to an older version, but there is no way
at the moment to globally pin tempest plugins to the last known
working version for a branch (but it will be added soon).

Change-Id: Iec75740e31e1be581ee0e7ef64214358ef6b790d

.zuul.yaml

@@ -116,6 +116,9 @@
       This is a base job for lvm with lio & tgt targets
     branches: ^(?=stable/(ocata|pike|queens|rocky|stein)).*$
     parent: cinder-tempest-plugin-lvm-barbican-base-abstract
+    required-projects:
+      - name: opendev.org/openstack/cinder-tempest-plugin
+        override-checkout: stein-last
     vars:
       tempest_test_blacklist: '{{ ansible_user_dir }}/{{ zuul.projects["opendev.org/openstack/tempest"].src_dir }}/tools/tempest-integrated-gate-storage-blacklist.txt'