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.

tools/venv3.sh

@@ -15,6 +15,7 @@ fi
   -e certbot-apache \
   -e certbot-dns-cloudflare \
   -e certbot-dns-digitalocean \
+  -e certbot-dns-google \
   -e certbot-nginx \
   -e letshelp-certbot \
   -e certbot-compatibility-test