acme: add StatusValid to expected values for Authorize

Let's Encrypt may respond with the "valid" value in status
field to authorization requests:
https://github.com/letsencrypt/boulder/blob/0543691d9e2cbcc1b94baf3b9fab4e64ca2750ef/ra/ra.go#L373-L402

The spec isn't clear on this:
https://tools.ietf.org/html/draft-ietf-acme-acme-02#section-6.4

Either way, this change follows common sense and should not produce
unexpected results.

Change-Id: Iac55aa1fb17d458f6d62e8da875ed86e3e4afbc3
Reviewed-on: https://go-review.googlesource.com/27432
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

acme/autocert/autocert.go

@@ -404,6 +404,11 @@ func (m *Manager) verify(ctx context.Context, domain string) error {
 	if err != nil {
 		return err
 	}
+	// maybe don't need to at all
+	if authz.Status == acme.StatusValid {
+		return nil
+	}
+
 	// pick a challenge: prefer tls-sni-02 over tls-sni-01
 	// TODO: consider authz.Combinations
 	var chal *acme.Challenge