Support accessing K8S API server via HTTPS_PROXY env vars

[chadlwilson]

Currently if you need to use some sort of proxy to access your K8S master/API server, eks-rolling-update doesn't seem to work. The kubectl bits work fine because kubectl seems to have support for these env vars out of the box.

With a bit of digging, this appears to be because the Kubernetes Python Client library does not support these env vars out of the box.

There is a suggested implementation at kubernetes-client/python#333 (comment) which could possible be sorted on the eks-rolling-update side to be able to support this.