internal/config: support alternative env vars
internal/config: support alternative env vars

If some GAE_XXX env vars are not defined, fall back to
GO_DISCOVERY_XXX.

This will be useful when we run in other environments, like GKE.

Change-Id: I56e9bd39e560c48329b4b03be8d06d94158d3823
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/250877
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite