Fix installing tempest plugins

When running stack.sh locally on stable branches
with tempest enabled and TEMPEST_PLUGINS set,
devstack will try to fetch master branch of requirements
and that fails if branch is not tracked.

Change-Id: Ia1ae6869a8fede2af5cd7c875e0946b6a75eb518
Closes-Bug: #1820051

Author: Vlad Gridin

lib/tempest

@@ -674,7 +674,7 @@ function install_tempest_plugins {
     pushd $TEMPEST_DIR
     if [[ $TEMPEST_PLUGINS != 0 ]] ; then
         # The requirements might be on a different branch, while tempest & tempest plugins needs master requirements.
-        (cd $REQUIREMENTS_DIR && git show master:upper-constraints.txt) > u-c-m.txt
+        (cd $REQUIREMENTS_DIR && git show origin/master:upper-constraints.txt) > u-c-m.txt
         tox -evenv-tempest -- pip install -c u-c-m.txt $TEMPEST_PLUGINS
         echo "Checking installed Tempest plugins:"
         tox -evenv-tempest -- tempest list-plugins