Add support to count in timeboard graphs about terraform-provider-datadog

[bkabrda]

Seems like this could be solved using the dynamic blocks feature that's been newly introduced in TF 0.12 [1]. I'll create a PR for our docs to present example usage.

[1] https://www.terraform.io/docs/configuration/expressions.html#dynamic-blocks

Count can't be applied to inner objects, so you'd need to create some object with a count and reference it in the timeboard resource. Offhand, I can't conceptualize how to make a data template work with this, but it might be doable.

I think creating a new graph data source could work, but that's a bit of a weird model because the resource ends up being just a local storage vehicle and not actually existing in the API.