Implement an Authenticator which can fulfill a dns-01 challenge using the
DigitalOcean API. Applicable only for domains using DigitalOcean for DNS.
Testing Done:
* `tox -e py27`
* `tox -e lint`
* Manual testing:
* Used `certbot certonly --dns-digitalocean -d`, specifying a
credentials file as a command line argument. Verified that a
certificate was successfully obtained without user interaction.
* Used `certbot certonly --dns-digitalocean -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 digitalocean 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).
* Credentials file missing token.
* Credentials file with blank token.
* Credentials file with incorrect token.
* Domain name not registered to DigitalOcean account.
