Fix crashes in rake tasks when a site has an empty cluster

Fixes 14659.

I reproduced the issues by following the instruction to add a new site here, and it quickly raised the crashes reported by @pijacquo during the installation of Toulouse.

lib/refrepo/gen/puppet/templates/conman.erb

@@ -30,7 +30,7 @@ server logfile="/var/log/conman.log"
 
   # Generate conman config for both cluster and server entries of the refapi
   site_refapi['servers'] ||= {}
-  cluster_list = site_refapi['clusters'].keys | site_refapi['servers'].keys | site_config.keys | site_credentials.keys
+  cluster_list = site_refapi.fetch('clusters', {}).keys | site_refapi['servers'].keys | site_config.keys | site_credentials.keys
   
   cluster_list.sort.each { |cluster_uid| 
     cluster_refapi      = site_refapi['clusters'][cluster_uid].fetch('nodes') rescue site_refapi['servers'][cluster_uid].fetch('nodes') rescue nil