acme/autocert: replace DNSNames with HostPolicy

Sanjay came up with this idea of a more flexible way
to place restrictions on the Manager using a HostPolicy hook
instead of the static DNSNames field.

HostPolicy allows for user-made custom policies,
as well as makes it possible to change the set of host names
dynamically, without restarting the Manager.

Change-Id: Ib7c6b047469edc6856b59c5e8365690e66f2a3a4
Reviewed-on: https://go-review.googlesource.com/27251
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed