Update bifrost-base Dockerfile.j2 template.

To facilitate build custimisations in template-overrides.j2
for bifrost-base, added a block section wrapping the execution
of the ansible install.yaml playbook.

Change-Id: I4eb18d03b7d7b2a2f65ec3c844af9149f9426949

docker/bifrost/bifrost-base/Dockerfile.j2

@@ -30,6 +30,7 @@ WORKDIR /bifrost
 RUN sed -e "s/.*mariadb.*//Ig" -i /etc/apt/sources.list && apt-get purge -y mariadb* mysql*
 {% endif %}
 
+{% block bifrost_ansible_install %}
 RUN bash -c './scripts/env-setup.sh && source ./env-vars && \
     ansible-playbook -vvvv -i /bifrost/playbooks/inventory/target /bifrost/playbooks/install.yaml \
     -e @/tmp/build_arg.yml && \
@@ -38,6 +39,7 @@ RUN bash -c './scripts/env-setup.sh && source ./env-vars && \
 {% else %}
     yum clean all'
 {% endif %}
+{% endblock %}
 {% endif %}
 
 COPY bifrost_sudoers /etc/sudoers.d/kolla_bifrost_sudoers