Add graph_facts variable to enable graphing (pie/bar) of Puppet facts

[bschonec]

Pull Request (PR) description

The extra_settings is somewhat cumbersome to use for GRAPH_FACTS in the settings.py file because it necessitates escaping the single quote character around the facts to be graphed. This PR adds the graph_facts variable as an array, sorts and unique-ifies the array and wraps the result in a set of single quotes:

GRAPH_FACTS = 'kernel,operatingsystem,operatingsystemmajrelease'