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
diff --git a/lib/horizon b/lib/horizon
index 4cabbe4..9c7ec00 100644
--- a/lib/horizon
+++ b/lib/horizon
@@ -81,11 +81,7 @@
     # 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 @@
         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