Allows full server role list to be sent to Datadog

As per, https://github.com/Shopify/ops/issues/904

This fork updates the datadog handler to use the full role list from the server, `node[:roles]` instead of just using the roles provided by the runlist `node.run_list`. Since its our fork we are also filtering out any namespaced roles that contain a double underscore.

I also namespaced the gem version to avoid conflicts with the rubygems versions.

Please review at your leisure, @wisq @coosh @kvs