Add optional openfaas to chart

[gabor-boros]

OpenFAAS allows a Lambda-like experience, running functions in a K8s cluster in pods. This allows providers and cluster operators to do many things such as exporting aggregated cluster or instance metrics to S3 or similar, triggering functions in the cluster that can manipulate resources, etc.

At OpenCraft we use it for exporting metrics, implementing Pull Request sandboxes, notify given groups upon a periodic build of the latest master branch of edx-platform failed, etc. This is something that would enable many opportunities for other providers as well, or K8s operators who have to maintain OpenEdX installations.