Disable Searchlight for CentOS 7/source

Searchlight is Python 3 while CentOS 7/source images are Python 2

Change-Id: I9d2e4f3630efad91b5cc94085ad1dd1f6680e017

docker/horizon/Dockerfile.j2

@@ -162,6 +162,10 @@ RUN ln -s horizon-source/* horizon \
     && cp -r /horizon/openstack_dashboard/conf/* /etc/openstack-dashboard/ \
     && cp /horizon/openstack_dashboard/local/local_settings.py.example /etc/openstack-dashboard/local_settings \
     && cp /horizon/manage.py /var/lib/kolla/venv/bin/manage.py \
+    {# NOTE(hrw) searchlight moved to Python 3 and CentOS 7 uses Python 2 #}
+    {% if base_package_type == 'rpm' and base_distro_tag.startswith('7') %}
+    && rm -rf /plugins/searchlight-ui* \
+    {% endif %}
     && if [ "$(ls /plugins)" ]; then \
            {{ macros.install_pip(horizon_plugins_pip_packages) }}; \
        fi \

kolla/image/build.py

@@ -143,6 +143,11 @@ UNBUILDABLE_IMAGES = {
     'centos': {
         "ovsdpdk",
     },
+
+    'centos+source': {
+        "searchlight-base",
+    },
+
     'debian': {
         "bifrost-base",  # tries to install 'mysql-server' which is not in
                          # Debian 'buster'