This change allows for more customizations when creating a tls-sni challenge response. Same reason as with https://go-review.googlesource.com/27750. Change-Id: Ia702ede2f4dd867814cfdc1f8925557d3eb455e9 Reviewed-on: https://go-review.googlesource.com/29053 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>