CI: add kolla_python_version variable

Change-Id: Ic697729bda80bfb83171bf68223a2703b8318aad

tests/templates/globals-default.j2

@@ -15,6 +15,10 @@ docker_custom_config:
   registry-mirrors:
     - {{ infra_dockerhub_mirror }}
 
+{% if kolla_python_version is defined and not is_previous_release %}
+distro_python_version: "{{ kolla_python_version }}"
+{% endif %}
+
 # Use a random router id, otherwise it may result in the same router id
 # in the CI gate.
 keepalived_virtual_router_id: "{{ 250 | random(1) }}"