Introduce a PYTHON env var

* $PYTHON will have the path to python runtime to be used
* Use $PYTHON to run all the scripts

Change-Id: Ib5ab7820fc18cae5e50ea47302b610494197ad47

functions-common

@@ -87,7 +87,7 @@ function write_clouds_yaml {
         CA_CERT_ARG="--os-cacert $SSL_BUNDLE_FILE"
     fi
     # demo -> devstack
-    $TOP_DIR/tools/update_clouds_yaml.py \
+    $PYTHON $TOP_DIR/tools/update_clouds_yaml.py \
         --file $CLOUDS_YAML \
         --os-cloud devstack \
         --os-region-name $REGION_NAME \
@@ -99,7 +99,7 @@ function write_clouds_yaml {
         --os-project-name demo
 
     # alt_demo -> devstack-alt
-    $TOP_DIR/tools/update_clouds_yaml.py \
+    $PYTHON $TOP_DIR/tools/update_clouds_yaml.py \
         --file $CLOUDS_YAML \
         --os-cloud devstack-alt \
         --os-region-name $REGION_NAME \
@@ -111,7 +111,7 @@ function write_clouds_yaml {
         --os-project-name alt_demo
 
     # admin -> devstack-admin
-    $TOP_DIR/tools/update_clouds_yaml.py \
+    $PYTHON $TOP_DIR/tools/update_clouds_yaml.py \
         --file $CLOUDS_YAML \
         --os-cloud devstack-admin \
         --os-region-name $REGION_NAME \

lib/horizon

@@ -81,11 +81,7 @@ function configure_horizon {
     # Horizon is installed as develop mode, so we can compile here.
     # Message catalog compilation is handled by Django admin script,
     # so compiling them after the installation avoids Django installation twice.
-    if python3_enabled; then
-        (cd $HORIZON_DIR; python${PYTHON3_VERSION} manage.py compilemessages)
-    else
-        (cd $HORIZON_DIR; python manage.py compilemessages)
-    fi
+    (cd $HORIZON_DIR; $PYTHON manage.py compilemessages)
 
     # ``local_settings.py`` is used to override horizon default settings.
     local local_settings=$HORIZON_DIR/openstack_dashboard/local/local_settings.py
@@ -166,11 +162,7 @@ function install_django_openstack_auth {
         git_clone_by_name "django_openstack_auth"
         # Compile message catalogs before installation
         _prepare_message_catalog_compilation
-        if python3_enabled; then
-            (cd $dir; python${PYTHON3_VERSION} setup.py compile_catalog)
-        else
-            (cd $dir; python setup.py compile_catalog)
-        fi
+        (cd $dir; $PYTHON setup.py compile_catalog)
         setup_dev_lib "django_openstack_auth"
     fi
     # if we aren't using this library from git, then we just let it

tools/install_prereqs.sh

@@ -83,6 +83,9 @@ fi
 
 if python3_enabled; then
     install_python3
+    export PYTHON=$(which python${PYTHON3_VERSION} 2>/dev/null || which python3 2>/dev/null)
+else
+    export PYTHON=$(which python 2>/dev/null)
 fi
 
 # Mark end of run