acme: use EC keys where possible

This change improves tests speed further by switching to EC keys where
possible.

We are also switching to EC-based token certs for TLS-SNI-x challenges
in this change. I've confirmed it works with Let's Encrypt production
servers as well, by fulfilling TLS-SNI-01 challenge for a test domain
using autocert.Manager.

Change-Id: I07c532d535ba413af97463dd97cca3d650dfccee
Reviewed-on: https://go-review.googlesource.com/27439
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed