use master upper-constraints when building tempest venv

The local requirements repo can be checked out to a stable branch,
in which case, the requirements might conflict with tempest's master
requirements.

Master branch's upper-constraints should be used when building tempest's
venv.

Closes-Bug: #1706009

Change-Id: Ifd64638cae2886671421149dbbff3a57f9c64257

Author: claudiubelu

lib/tempest

@@ -551,7 +551,10 @@ function configure_tempest {
     if [[ "$OFFLINE" != "True" ]]; then
         tox -revenv-tempest --notest
     fi
-    tox -evenv-tempest -- pip install -c $REQUIREMENTS_DIR/upper-constraints.txt -r requirements.txt
+
+    # The requirements might be on a different branch, while tempest needs master requirements.
+    git -C $REQUIREMENTS_DIR show master:upper-constraints.txt > u-c-m.txt
+    tox -evenv-tempest -- pip install -c u-c-m.txt -r requirements.txt
 
     # Auth:
     iniset $TEMPEST_CONFIG auth tempest_roles "Member"