Enumerate loaded hosts and check for datadog_api_key in their vars

[martinskis]

The datadog_api_key var is only checked if it is set for localhost. Which didn't really fit an inventory with multistage environment such as https://www.digitalocean.com/community/tutorials/how-to-manage-multistage-environments-with-ansible where we don't want to log dev runs.

I expected to be able to define it in group_vars, not sure if it's the right way, since it's a callback plugin that needs to be defined on the host that runs the playbook only.