Support Datadog with Fargate

So its been a while since i submitted this and I figure I should get off my butt and fix my own problems… So if I was to implement this feature myself so I can offer it as a PR on the rack repo, is there any particular objection to a fargate specific configuration option?

My current thought is adding an array to convox.yml under/in the services: yaml dict, called something like fargate-sidecars that would be ignored if the Fargate template parameter is false and would connect some config options, to a section of the cloudformation template service.json.tmpl that contains the necessary for correctly configuring it the way DataDog recommend.

Any advice/input from @ddollar / @ed_convox / other Convox people, would be most appreciated.