stack/erp5: keep template-apache-backend-conf for slapos-master

Eventhough software/erp5 no longer uses this template, it is still used by
software/slapos-master, so keep the entry point that will be empty for ERP5,
but set only in SlapOS Master.

stack/erp5/buildout.cfg

@@ -152,6 +152,10 @@ link-binary =
 [template-run-zelenium]
 < = download-base
 
+[template-apache-backend-conf]
+# BBB kept for compatibility with software/slapos-master
+recipe =
+target = not used
 
 [template]
 recipe = slapos.recipe.template:jinja2
@@ -180,6 +184,7 @@ context =
     key xz_utils_location xz-utils:location
     key haproxy_location haproxy:location
     key rsyslogd_location rsyslogd:location
+    key template_apache_conf template-apache-backend-conf:target
     key instance_common_cfg instance-common:rendered
     key jsl_location jsl:location
     key jupyter_enable_default erp5-defaults:jupyter-enable-default