Allow configuring environment variables on controller container in Helm chart

[adnowakodbx1]

In our environment internet traffic (including traffic to Buildkite) needs to go through proxy. To make that happen we need to configure proxy environment variables on controller container. Also, because of how our k8s cluster is configured we also need to override KUBERNETES_SERVICE_HOST environment variable in controller container so that it can reach correct endpoint of k8s API server.

Can property for appending environment variables to controller container be added to Helm chart values?

[DrJosh9000]

Thanks for raising this @adnowakodbx1 ! If you're keen, we'd be happy to review a PR implementing this.