Added Python-2.6 to the classifier.

OpenStack is often used with dustributions of GNU/Linux
which use python-2.6 as a default python interpreter.
Other OpenStack projects declare compatibility with python-2.6.

Ironic is been continuously tested for compatibility with
python-2.6 so we must declare this in the classifier to
stay synced with other OpenStack services.

Change-Id: Idf296e925c39d2847e7c8d7bfca073b5f9681754
Merge "Added Python-2.6 to the classifier."