apache-frontend: Avoid failing if no wildcard match

Since using apache-2.4[1] the Include directive will fail if no glob
matches, this switch to new, IncludeOptional[2], available since 2.4, in
order to not fail if no configurations are present.

 [1] 1f7bf3d1
 [2] http://httpd.apache.org/docs/2.4/mod/core.html#includeoptional

software/apache-frontend/buildout.hash.cfg

@@ -39,7 +39,7 @@ md5sum = 665e83d660c9b779249b2179d7ce4b4e
 
 [template-apache-frontend-configuration]
 filename = templates/apache.conf.in
-md5sum = 82cdb4ab02fec36285b9c1ce502f82f0
+md5sum = a56045e7b53ff00ab34d2a8f911fc1a1
 
 [template-custom-slave-list]
 filename = templates/apache-custom-slave-list.cfg.in

software/apache-frontend/templates/apache.conf.in

@@ -166,8 +166,8 @@ SSLProxyCheckPeerCN off
 SSLProxyCheckPeerExpire off
 
 include {{frontend_configuration.get('log-access-configuration')}}
-include {{ slave_configuration_directory }}/*.conf
-include {{ slave_with_cache_configuration_directory }}/*.conf
+includeoptional {{ slave_configuration_directory }}/*.conf
+includeoptional {{ slave_with_cache_configuration_directory }}/*.conf
 
 ErrorDocument 404 /notfound.html
 RewriteRule (.*) /notfound.html [R=404,L]