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/selection.py

@@ -165,7 +165,8 @@ def choose_plugin(prepared, question):
 
 noninstaller_plugins = ["webroot", "manual", "standalone", "dns-cloudflare", "dns-cloudxns",
                         "dns-digitalocean", "dns-dnsimple", "dns-dnsmadeeasy", "dns-google",
-                        "dns-linode", "dns-luadns", "dns-nsone", "dns-rfc2136", "dns-route53"]
+                        "dns-linode", "dns-luadns", "dns-nsone", "dns-rfc2136", "dns-route53",
+                        "dns-sakuracloud"]
 
 def record_chosen_plugins(config, plugins, auth, inst):
     "Update the config entries to reflect the plugins we actually selected."
@@ -300,6 +301,8 @@ def cli_plugin_requests(config):  # pylint: disable=too-many-branches
         req_auth = set_configurator(req_auth, "dns-rfc2136")
     if config.dns_route53:
         req_auth = set_configurator(req_auth, "dns-route53")
+    if config.dns_sakuracloud:
+        req_auth = set_configurator(req_auth, "dns-sakuracloud")
     logger.debug("Requested authenticator %s and installer %s", req_auth, req_inst)
     return req_auth, req_inst