code: def once (key, store: "/var/cache/blower.json", quiet: false)
        return yield unless key
        log.info "once: #{key}", quiet: quiet do
          hash_map(hosts) do |host|
            done = begin
              JSON.parse(host.read(store, quiet: true))
            rescue => e
              {}
            end
            unless done[key]
              on [host] do
                yield
              end
              done[key] = true
              host.write(done.to_json, store, quiet: true)
            end
          end
        end
      end
docs: Execute a block only once per host.
   It is usually preferable to make tasks idempotent, but when that isn't
   possible, +once+ will only execute the block on hosts where a block
   with the same key hasn't previously been successfully executed.
   @param [String] key Uniquely identifies the block.
   @param [String] store File to store +once+'s state in.
   @macro quietable