caddy-frontend: Protect against losing custom_domain

In some cases domain can come from "outside" of the profile, and be filled
with "garbage", so if custom_domain is set, do not overwrite it.

software/caddy-frontend/buildout.hash.cfg

@@ -30,7 +30,7 @@ md5sum = 99ec567c429ff82571d08818eaaed390
 
 [template-slave-list]
 filename = templates/apache-custom-slave-list.cfg.in
-md5sum = d2e0132dded2f65e3590c72a154f6dd6
+md5sum = 3faad78c80b0e9235849f0b5346eebbc
 
 [template-slave-configuration]
 filename = templates/custom-virtualhost.conf.in

software/caddy-frontend/templates/apache-custom-slave-list.cfg.in

@@ -117,7 +117,9 @@ output = {{ plugin_directory }}/${:name}
 
 {%   if enable_cache and 'url' in slave_instance %}
 {%     if 'domain' in slave_instance %}
-{%       do slave_instance.__setitem__('custom_domain', slave_instance.get('domain')) %}
+{%       if not slave_instance.get('custom_domain') %}
+{%         do slave_instance.__setitem__('custom_domain', slave_instance.get('domain')) %}
+{%       endif %}
 {%     endif %}
 {%     do slave_instance.__setitem__('backend_url', slave_instance.get('url')) %}
 {%     do slave_instance.__setitem__('https_backend_url', slave_instance.get('https-url', slave_instance.get('url'))) %}