Pin tempest and tempest-horizon in stein

stable/stein is in the extended maintenance phase and we will pin
tempest version to the latest tempest release (26.0.0) in Wallaby.
tempest 26.0.0 was released before merging tempest-horizon back,
so we need tempest-horizon too. This commit tries to install
tempest 26.0.0 and tempest-horizon 1.1.0 (latest release).
It makes the merge of tempest-horizon more smooth.

Change-Id: I5f2369e47cfaa9ccfdf8aa1f91348349c55b8515

.zuul.yaml

@@ -47,7 +47,6 @@
     required-projects:
       - name: openstack/horizon
       - name: openstack/tempest
-      - name: openstack/tempest-horizon
     irrelevant-files:
       - ^.*\.rst$
       - ^doc/.*$
@@ -63,7 +62,8 @@
       - ^openstack_dashboard/dashboards/.*/tests.py$
     vars:
       devstack_localrc:
-        TEMPEST_PLUGINS: "{{ ansible_user_dir }}/src/opendev.org/openstack/tempest-horizon"
+        TEMPEST_BRANCH: "26.0.0"
+        TEMPEST_PLUGINS: "tempest-horizon==1.1.0"
         USE_PYTHON3: true
       devstack_services:
         horizon: true