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/functions-common b/functions-common
index 8d03b88..8d32bb4 100644
--- a/functions-common
+++ b/functions-common
@@ -87,7 +87,7 @@
         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 @@
         --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 @@
         --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 \