acme: add WithTemplate option for tls-sni certs

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>
2 files changed