Inconsistent usage of DATADOG_API_KEY and DD_API_KEY

[albertvaka]

Most Datadog products (Agent, Tracers, etc.) use DD_API_KEY as the environment variable for the API key.

In datadog-ci, we mostly use DATADOG_API_KEY while some commands accept both DATADOG_API_KEY and DD_API_KEY.

For consistency, I think we should accept DD_API_KEY in all commands. We probably want to still accept DATADOG_API_KEY for backwards compatibility, but I think we should prefer DD_API_KEY instead (eg: in documentation).

The same applies to DATADOG_SITE vs DD_SITE.