def pubsub(domain)
      host = @vhosts.values.find {|host| host.pubsub?(domain) }
      host.pubsubs[domain.to_s] if host
    end
def pubsub(domain)
      host = @vhosts.values.find {|host| host.pubsub?(domain) }
      host.pubsubs[domain.to_s] if host
    end
Returns the PubSub system for the domain or nil if pubsub is not enabled
 for this domain.