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.

certbot/plugins/disco.py

@@ -36,6 +36,7 @@ class PluginEntryPoint(object):
         "certbot-dns-nsone",
         "certbot-dns-rfc2136",
         "certbot-dns-route53",
+        "certbot-dns-sakuracloud",
         "certbot-nginx",
         "certbot-postfix",
     ]