allow override of kubelet host with KUBERNETES_KUBELET_HOST env var

[stonith]

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

What does this PR do?

dd-agent assumes that the kubelet API is available at the default gateway of the container. This PR allows overriding of the kubelet host if the KUBERNETES_KUBELET_HOST environment variable is found. As an example, the kubernetes downward API in 1.4+ can be used to specify the node name:

          - name: KUBERNETES_KUBELET_HOST
            valueFrom:
              fieldRef:
                fieldPath: spec.nodeName

Motivation

There are implementations of kubernetes where the kubelet API is not available at the default gateway. An example of this would be when Calico is used: projectcalico/calico-containers#1099

What inspired you to submit this pull request?

We're testing Calico with kubernetes and a docker-dd-agent daemonset does not function properly.

Additional Notes

I can cut a PR to update docs in DataDog/docker-dd-agent if this gets merged.

Anything else we should know when reviewing?

• this addresses kubernetes environments where the kubelet api is
  not reachable at the default gateway
• addresses projectcalico/calico-containers#1099

[caseydavenport]

@stonith Just to clarify my understanding:

fieldPath: spec.nodeName

means that the node's name in Kubernetes needs to be resolvable by the pod (or a valid IP address for that node), correct?

[stonith]

@caseydavenport in this example yes, the pod would have to be able to resolve the node's name.

[caseydavenport]

@stonith cool, thanks! This looks good to me.

[remh]

Thanks! This looks great to me. @hkaj can you review as well ?

[hkaj]

Thanks @stonith. It looks good to me too, I'll probably merge it today.
As you mentioned we'll also need a PR in docker-dd-agent to add a mention of it in here: https://github.com/DataDog/docker-dd-agent/blob/master/README.md#environment-variables-1

Mind taking care of it too @stonith ?
Thanks again!

[hkaj]

Just tested, all good.

[stonith]

PR for the README.

NOTE:
When trying to implement this with CNI networking I ran into kubernetes/kubernetes#31307 which unfortunately means hostPort daemonset with my above example doesn't currently work. A NodePort service should work though.