Add a variable to configure the Tempest venv upper constraints

We use Tempest master for testing the supported stable
branches so using master upper constraints works fine but
when we need to use old Tempest in the below cases then master
upper constraints do not work and devstack will not be
able to install Tempest in vnenv:

- Testing Extended Maintenance branch
- Testing py2.7 jobs until stable/train with in-tree tempest plugins

This commit adds a variable to set the compatible upper constraint
to use for Tempest's old version.

Few of the current failure which can be fixed by this new configurable var:
- networking-generic-switch-tempest-dlm-python2
  - https://zuul.opendev.org/t/openstack/build/ebcf3d68d62c4af3a43a222aa9ce5556
- devstack-platform-xenial on stable/steinand stable/train
  - https://zuul.opendev.org/t/openstack/build/37ffc1af6f3f4b44b5ca8cbfa27068ac

Change-Id: I5b2217d85e6871ca3f7a3f6f859fdce9a50d3946
(cherry picked from commit 3bdc8f66ad)

lib/tempest

@@ -110,6 +110,21 @@ function image_size_in_gib {
     echo $size | python3 -c "import math; print(int(math.ceil(float(int(input()) / 1024.0 ** 3))))"
 }
 
+function set_tempest_venv_constraints {
+    local tmp_c
+    tmp_c=$1
+    if [[ $TEMPEST_VENV_UPPER_CONSTRAINTS == "master" ]]; then
+        (cd $REQUIREMENTS_DIR && git show origin/master:upper-constraints.txt) > $tmp_c
+    else
+        echo "Using $TEMPEST_VENV_UPPER_CONSTRAINTS constraints in Tempest virtual env."
+        cat $TEMPEST_VENV_UPPER_CONSTRAINTS > $tmp_c
+        # NOTE: setting both tox env var and once Tempest start using new var
+        # TOX_CONSTRAINTS_FILE then we can remove the old one.
+        export UPPER_CONSTRAINTS_FILE=$TEMPEST_VENV_UPPER_CONSTRAINTS
+        export TOX_CONSTRAINTS_FILE=$TEMPEST_VENV_UPPER_CONSTRAINTS
+    fi
+}
+
 # configure_tempest() - Set config files, create data dirs, etc
 function configure_tempest {
     if [[ "$INSTALL_TEMPEST" == "True" ]]; then
@@ -614,10 +629,9 @@ function configure_tempest {
         tox -revenv-tempest --notest
     fi
 
-    # The requirements might be on a different branch, while tempest needs master requirements.
     local tmp_u_c_m
     tmp_u_c_m=$(mktemp -t tempest_u_c_m.XXXXXXXXXX)
-    (cd $REQUIREMENTS_DIR && git show origin/master:upper-constraints.txt) > $tmp_u_c_m
+    set_tempest_venv_constraints $tmp_u_c_m
     tox -evenv-tempest -- pip install -c $tmp_u_c_m -r requirements.txt
     rm -f $tmp_u_c_m
 
@@ -740,12 +754,17 @@ function install_tempest {
     # TEMPEST_DIR already exist until RECLONE is true.
     git checkout $TEMPEST_BRANCH
 
+    local tmp_u_c_m
+    tmp_u_c_m=$(mktemp -t tempest_u_c_m.XXXXXXXXXX)
+    set_tempest_venv_constraints $tmp_u_c_m
+
     tox -r --notest -efull
     # NOTE(mtreinish) Respect constraints in the tempest full venv, things that
     # are using a tox job other than full will not be respecting constraints but
     # running pip install -U on tempest requirements
-    $TEMPEST_DIR/.tox/tempest/bin/pip install -c $REQUIREMENTS_DIR/upper-constraints.txt -r requirements.txt
+    $TEMPEST_DIR/.tox/tempest/bin/pip install -c $tmp_u_c_m -r requirements.txt
     PROJECT_VENV["tempest"]=${TEMPEST_DIR}/.tox/tempest
+    rm -f $tmp_u_c_m
     popd
 }
 
@@ -753,10 +772,9 @@ function install_tempest {
 function install_tempest_plugins {
     pushd $TEMPEST_DIR
     if [[ $TEMPEST_PLUGINS != 0 ]] ; then
-        # The requirements might be on a different branch, while tempest & tempest plugins needs master requirements.
         local tmp_u_c_m
         tmp_u_c_m=$(mktemp -t tempest_u_c_m.XXXXXXXXXX)
-        (cd $REQUIREMENTS_DIR && git show origin/master:upper-constraints.txt) > $tmp_u_c_m
+        set_tempest_venv_constraints $tmp_u_c_m
         tox -evenv-tempest -- pip install -c $tmp_u_c_m $TEMPEST_PLUGINS
         rm -f $tmp_u_c_m
         echo "Checking installed Tempest plugins:"

stackrc

@@ -298,6 +298,7 @@ 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}
 
 
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