Google Cloud DNS Authenticator (#4581)

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

Testing Done:
 * `tox -e py27`
 * `tox -e lint`
 * Manual testing:
    * Used `certbot certonly --dns-google -d`, specifying a credentials
      file as a command line argument. Verified that a certificate was
      successfully obtained without user interaction.
    * Used `certbot certonly --dns-google -d`, without specifying a
      credentials file as a command line argument. Verified that the
      user was prompted and that a certificate was successfully
      obtained.
    * Used `certbot certonly -d`. Verified that the user was prompted for
      a credentials file after selecting google interactively and that
      a certificate was successfully obtained.
    * Used `certbot renew --force-renewal`. Verified that certificates
      were renewed without user interaction.
 * Negative testing:
    * Path to non-existent credentials file.
    * Credentials file with unsafe permissions (644).
    * Domain name not registered to Google Cloud Platform account.

certbot-dns-digitalocean/certbot_dns_digitalocean/dns_digitalocean.py

@@ -26,7 +26,7 @@ class Authenticator(dns_common.DNSAuthenticator):
         self.credentials = None
 
     @classmethod
-    def add_parser_arguments(cls, add):
+    def add_parser_arguments(cls, add):  # pylint: disable=arguments-differ
         super(Authenticator, cls).add_parser_arguments(add)
         add('credentials', help='DigitalOcean credentials INI file.')