Gehirn Infrastracture Service DNS Authenticator (#5702)

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.

tools/venv3.sh

@@ -18,6 +18,7 @@ fi
   -e certbot-dns-digitalocean \
   -e certbot-dns-dnsimple \
   -e certbot-dns-dnsmadeeasy \
+  -e certbot-dns-gehirn \
   -e certbot-dns-google \
   -e certbot-dns-linode \
   -e certbot-dns-luadns \