Use the env var KUBERNETES_SERVICE_HOST if present

[arohner]

Note: Please remember to review the Datadog Contribution Guidelines
if you have not yet done so.

What does this PR do?

In the kubernetes check, use the env var KUBERNTES_SERVICE_HOST to specify the API endpoint, if present.

Motivation

See #2551 (comment) . When using dd-agent in namespaces such as kube-system, the kube-dns service might not be running, so the kubernetes host might not be present.

[hkaj]

Thanks @arohner ! You're right, we should avoid relying on extensions as much as possible.