Test-infra: prow: Github's API endpoint is not configurable

[fen4o]

Right now GitHub's API endpoint is hardcoded to https://api.github.com. This works great for the public GitHub, but for the Enterprise GitHub offering, the endpoint is custom.

Mungegithub supports GitHub Endpoint configuration via CLI flag.

Both hook and plank talk to GitHub's API. I guess that there are two options for configuration:
CLI flag to both binaries or Prow's config.yaml file.

I would go for the config.yaml, because later, more Github client options can be configured without the need to add CLI flags to the binaries.

It would be great if someone can provide some suggestions and ideas before I create a PR for this issue.

[kargakis]

/area prow

[spxtr]

I normally try to limit CLI flags, but for this case I would be okay with one. We can make the default https://api.github.com so that most users do not need to set it at all. I would suggest this.

A config option is also fine, although I'm not sure we ever want to allow configuring the other options you linked.

[caiwei-ebay]

https://github.com/kubernetes/test-infra/blob/master/prow/git/git.go#L85

This file is still referencing "github.com" as default base.

[HerrmannHinz]

any update? GHE would be super nice.