def cluster(&block)
      cluster = Cluster.new(self, :logger => logger)
      block ? block.call(cluster) : cluster
    end
def cluster(&block)
      cluster = Cluster.new(self, :logger => logger)
      block ? block.call(cluster) : cluster
    end
Returns the Stretcher::Cluster for this server
 Optionally takes a block, which will be passed a single arg with the
 Cluster object.  The block returns the evaluated value of the block.