make use-forwarded-headers a configuration option

[LP]

With nginx ingress controller version 0.22.0 [1], a default of 'false' for use-forwarded-headers was introduced. This is quite a breaking change for clusters with external L7 LBs. I don't consider this configuration to be too exotic.

If we set this in the configuration map of the nginx controller, it'll get overwritten by the next charm update [2]. Please make this a charm option in order to persist it.

[1] https://github.com/kubernetes/ingress-nginx/releases/tag/nginx-0.22.0
[2] https://github.com/charmed-kubernetes/charm-kubernetes-worker/blob/master/reactive/kubernetes_worker.py#L138

[Tim Van Steenburgh (tvansteenburgh)]

PR: https://github.com/charmed-kubernetes/charm-kubernetes-worker/pull/54