Add option for propagating environment variables

[psibi]

Right now, kubel works when the apropriate credential is part of ~/.kube. But it doesn't work,if I have KUBECONFIG environment variable pointing out to somewhere else (basically if it's not globally set). If you use something like direnv to have project specific environment variable, kubel doesn't work there. And at work, I usually have separate clusters each being managed in their own directory.

This PR adds supports for propagating environment variables and making kubel working in those environment. For EKS cluster, I have to often propagate some extra environment variables to make it work with kubel:

(kubel-env-variables (list "KUBECONFIG" "AWS_ACCESS_KEY_ID" "AWS_SECRET_ACCESS_KEY"))

[abrochard]

is this the only place where you need to do that? what about kubel--exec?

[psibi]

This was the only place where it was required for me to unblock it and have a working kubel version in my envrc based setup.

Note that it still isn't perfect and has some shortcomings which I plan to address in a subsequent PR. But I believe those changes are going to be slightly big (I have a working copy locally, but I'm still testing out various scenarios).

[abrochard]

I suspect things are going to be very wonky if the changes are only applied to this function and not also kubel--exec

[abrochard]

Thank you for submitting this! I left a few comments. Could you also please update the doc to show intended usage?