add flexible configuration through node attributes

you would use this to specify configurations through chef that are not
currently achievable through chef configuration, for example, in a role:

    default_attributes(
      "datadog" => {
        "conf_lines" => {
          "memcache_instance_1" => "localhost:1234:my_tag"
        }
      }
    )

Caveat emptor: this makes it possible to specify invalid, possibly unparseable configuration.