cmd/coordinator: add LetsEncrypt support

The buildlets were partially updated to support this in
https://golang.org/cl/38792 but overlooked that ServerName == "go" was
still hard-coded. This CL also fixes that in the buildlet.

Fixes golang/go#16442

Change-Id: Ia2b794bdf9df8ab75875b9951b53a7bb5f5f6afe
Reviewed-on: https://go-review.googlesource.com/38798
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
6 files changed