Sakura Cloud DNS Authenticator (#5701)

Implement an Authenticator which can fulfill a dns-01 challenge using
the Sakura Cloud DNS API.
Applicable only for domains using Sakura Cloud for DNS.

Testing Done:
 * `tox -e py27`
 * `tox -e lint`
 * Manual testing:
    * Used `certbot certonly --dns-sakuracloud -d`, specifying a
      credentials file as a command line argument. Verified that a
      certificate was successfully obtained without user interaction.
 * Negative testing:
    * Path to non-existent credentials file.
    * Credentials file with unsafe permissions (644).
    * Domain name not registered to Sakura Cloud account.

tools/venv3.sh

@@ -23,6 +23,7 @@ fi
   -e certbot-dns-luadns \
   -e certbot-dns-nsone \
   -e certbot-dns-route53 \
+  -e certbot-dns-sakuracloud \
   -e certbot-nginx \
   -e certbot-postfix \
   -e letshelp-certbot \