Update openvswitch base Dockerfile to include footers

This is not in sync with the current changes being
merged for customization

Change-Id: I7779a52cdeea9a4b6f7580388b86faa1d8c673eb
Partially-implements: blueprint third-party-plugin-support

docker/openvswitch/openvswitch-base/Dockerfile.j2

@@ -15,3 +15,6 @@ COPY extend_start.sh /usr/local/bin/kolla_extend_start
 
 RUN touch /usr/local/bin/kolla_openvswitch_extend_start \
     && chmod 755 /usr/local/bin/kolla_extend_start /usr/local/bin/kolla_openvswitch_extend_start
+
+{% block openvswitch_base_footer %}{% endblock %}
+{% block footer %}{% endblock %}