Pin Tempest to 26.1.0 tag for stable/train testing

Stable train is in Extended maintenance state[1] and
we need to cap stable/train with older Tempest.

But because of oslo.utils version bump in Tempest 27.0.0
and 28.0.0 which is not compatible with stable/train
upper constraints so these versions do not work in stable/train

- https://zuul.opendev.org/t/openstack/build/1146c9ce9f474d0d82b99b48b61c531e

Let's use the Tempest 26.1.0 which is also compatible with stable/train
testing.

Related-Bug: #1955418
Change-Id: I376be5f5a4d693aeaaf74ca5285269f796103c15

Author: Ghanshyam Mann

stackrc

@@ -314,8 +314,10 @@ REQUIREMENTS_BRANCH=${REQUIREMENTS_BRANCH:-$TARGET_BRANCH}
 
 # Tempest test suite
 TEMPEST_REPO=${TEMPEST_REPO:-${GIT_BASE}/openstack/tempest.git}
-TEMPEST_BRANCH=${TEMPEST_BRANCH:-$BRANCHLESS_TARGET_BRANCH}
-TEMPEST_VENV_UPPER_CONSTRAINTS=${TEMPEST_VENV_UPPER_CONSTRAINTS:-master}
+# NOTE (gmann): using 26.1.0 as that is compatible with stable/train
+# constraints. We are not using 28.0.0 or 27.0.0 due to bug#1955418
+TEMPEST_BRANCH=${TEMPEST_BRANCH:-26.1.0}
+TEMPEST_VENV_UPPER_CONSTRAINTS=${TEMPEST_VENV_UPPER_CONSTRAINTS:-$REQUIREMENTS_DIR/upper-constraints.txt}
 
 
 ##############

tests/test_refs.sh

@@ -15,7 +15,7 @@
 
 echo "Ensuring we don't have crazy refs"
 
-REFS=`grep BRANCH stackrc | grep -v 'TARGET_BRANCH' | grep -v 'NOVNC_BRANCH'`
+REFS=`grep BRANCH stackrc | grep -v 'TARGET_BRANCH' | grep -v 'NOVNC_BRANCH' | grep -v 'TEMPEST_BRANCH'`
 rc=$?
 if [[ $rc -eq 0 ]]; then
     echo "Branch defaults must be one of the *TARGET_BRANCH values. Found:"