* Drop support for EOL Python 2.6
* Use more helpful assertIn/NotIn instead of assertTrue/False
* Drop support for EOL Python 3.3
* Remove redundant Python 3.3 code
* Restore code for RHEL 6 and virtualenv for Py2.7
* Revert pipstrap.py to upstream
* Merge py26_packages and non_py26_packages into all_packages
* Revert changes to *-auto in root
* Update by calling letsencrypt-auto-source/build.py
* Revert permissions for pipstrap.py
Implement an Authenticator which can fulfill a dns-01 challenge using
the CloudXNS DNS API. Applicable only for domains using CloudXNS DNS.
Testing Done:
* `tox -e py27`
* `tox -e lint`
* Manual testing:
* Used `certbot certonly --dns-cloudxns -d`, specifying a
credentials file as a command line argument. Verified that a
certificate was successfully obtained without user interaction.
* Used `certbot certonly --dns-cloudxns -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 cloudxns 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 CloudXNS account.
