* Refactor tests out of module for certbot-dns-cloudflare
* Refactor tests out of module for certbot-dns-cloudxns
* Refactor tests out of module for certbot-dns-digitalocean
* Refactor tests out of module for certbot-dns-dnsimple
* Refactor tests out of module for certbot-dns-dnsmadeeasy
* Refactor tests out of module for certbot-dns-gehirn
* Refactor tests out of module for certbot-dns-google
* Refactor tests out of module for certbot-dns-linode
* Refactor tests out of module for certbot-dns-luadns
* Refactor tests out of module for certbot-dns-nsone
* Refactor tests out of module for certbot-dns-ovh
* Refactor tests out of module for certbot-dns-rfc2136
* Refactor tests out of module for certbot-dns-sakuracloud
* Refactor tests out of module for certbot-dns-route53
* Move certbot-dns-google testdata/ under tests/
* Use pytest for dns plugins
* Exclude pycache and .py[cod]
Implement an Authenticator which can fulfill a dns-01 challenge using
the Gehirn DNS (Gehirn Infrastructure Service) API.
Applicable only for domains using Gehirn DNS for DNS.
Testing Done:
* `tox -e py27`
* `tox -e lint`
* Manual testing:
* Used `certbot certonly --dns-gehirn -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 Gehirn DNS account.
