Implement an Authenticator which can fulfill a dns-01 challenge using the OVH DNS API. Applicable only for domains using OVH DNS.
Testing Done:
* `tox -e py27`
* `tox -e lint`
* Manual testing:
* Used `certbot certonly --dns-ovh -d`, specifying a credentials file as a command line argument. Verified that a certificate was successfully obtained without user interaction.
* Used `certbot certonly --dns-ovh -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 dnsimple 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).
* Path to credentials file with an invalid application key.
* Path to credentials file with an invalid application secret.
* Path to credentials file with an invalid consumer key.
* Path to credentials file with missing properties.
* Domain name not registered to OVH account.
