CI: Fix prometheus-opensearch-upgrade CI job

The upgrade job needs the haproxy exporter group, which
was missing from the inventory.

Change-Id: Ie4ecf283a2f4ac056ace5e76f2acc4ba1a8fe0b4

tests/templates/inventory.j2

@@ -681,6 +681,12 @@ storage
 [prometheus-mysqld-exporter:children]
 mariadb
 
+# TODO(dougszu): Remove in D cycle
+{% if is_upgrade %}
+[prometheus-haproxy-exporter:children]
+loadbalancer
+{% endif %}
+
 [prometheus-memcached-exporter:children]
 memcached