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.

tox.ini

@@ -25,7 +25,8 @@ dns_packages =
     certbot-dns-luadns \
     certbot-dns-nsone \
     certbot-dns-rfc2136 \
-    certbot-dns-route53
+    certbot-dns-route53 \
+    certbot-dns-sakuracloud
 all_packages =
     acme[dev] \
     .[dev] \
@@ -52,6 +53,7 @@ source_paths =
     certbot-dns-nsone/certbot_dns_nsone
     certbot-dns-rfc2136/certbot_dns_rfc2136
     certbot-dns-route53/certbot_dns_route53
+    certbot-dns-sakuracloud/certbot_dns_sakuracloud
     certbot-nginx/certbot_nginx
     certbot-postfix/certbot_postfix
     letshelp-certbot/letshelp_certbot